diff --git a/.dir-locals.el b/.dir-locals.el
new file mode 100644
index 0000000..1909c38
--- /dev/null
+++ b/.dir-locals.el
@@ -0,0 +1,2 @@
+((nil . ((eval . (flycheck-mode 0))
+         (eval . (rainbow-mode 0)))))
diff --git a/assert.c b/assert.c
new file mode 100644
index 0000000..03549cf
--- /dev/null
+++ b/assert.c
@@ -0,0 +1,11 @@
+void assert_type (Ast_Node* node, Ast_Node_Type type) {
+    if (node->type == type) return;
+
+    char *wanted, *got, *message;
+    wanted = Ast_Node_Type_to_string(type);
+    got = Ast_Node_Type_to_string(node->type);
+    asprintf(&message, "Type assertion failed:\n\t"
+             "Wanted: %s\n\t"
+             "Got   : %s\n", wanted, got);
+    panic(message);
+}
diff --git a/ast.c b/ast.c
index cc1a494..b2c916f 100644
--- a/ast.c
+++ b/ast.c
@@ -4,18 +4,23 @@ typedef enum {
     Ast_Node_Type_Nil,
 
     Ast_Node_Type_Symbol,
+    Ast_Node_Type_Keyword,
     Ast_Node_Type_Number,
     Ast_Node_Type_String,
 
     Ast_Node_Type_Cons_Cell,
     Ast_Node_Type_Function,
     Ast_Node_Type_Built_In_Function,
-} Ast_Type;
+} Ast_Node_Type;
 
 typedef struct {
     char* identifier;
 } Symbol;
 
+typedef struct {
+    char* identifier;
+} Keyword;
+
 typedef struct {
     double value;
 } Number;
@@ -44,9 +49,10 @@ typedef struct {
 } Built_In_Function;
 
 struct Ast_Node {
-    Ast_Type type;
+    Ast_Node_Type type;
     union {
         Symbol*    symbol;
+        Keyword*   keyword;
         Number*    number;
         String*    string;
         Cons_Cell* cons_cell;
@@ -73,6 +79,13 @@ Ast_Node* create_ast_node_number(double number) {
     return node;
 }
 
+Ast_Node* create_ast_node_string(char* str) {
+    Ast_Node* node = new(Ast_Node);
+    node->type = Ast_Node_Type_String;
+    node->value.string = new(String);
+    node->value.string->value = str;
+    return node;
+}
 
 Ast_Node* create_ast_node_symbol(char* identifier) {
     Ast_Node* node = new(Ast_Node);
diff --git a/built_ins.c b/built_ins.c
index 8996c2d..c98c560 100644
--- a/built_ins.c
+++ b/built_ins.c
@@ -1,7 +1,7 @@
 Ast_Node* built_in_add(Ast_Node* operands) {
     double sum = 0;
     while (operands->type == Ast_Node_Type_Cons_Cell) {
-        // TODO(Felix): assert type
+        assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
         sum += operands->value.cons_cell->first->value.number->value;
         operands = operands->value.cons_cell->rest;
     }
@@ -10,12 +10,43 @@ Ast_Node* built_in_add(Ast_Node* operands) {
 }
 
 Ast_Node* built_in_substract(Ast_Node* operands) {
-    double diff = operands->value.cons_cell->first->value.number->value;
+    assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
+    double difference = operands->value.cons_cell->first->value.number->value;
+
+    operands = operands->value.cons_cell->rest;
+    while (operands->type == Ast_Node_Type_Cons_Cell) {
+        assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
+
+        difference -= operands->value.cons_cell->first->value.number->value;
+        operands = operands->value.cons_cell->rest;
+    }
+    return create_ast_node_number(difference);
+}
+
+Ast_Node* built_in_multiply(Ast_Node* operands) {
+    assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
+    double product = operands->value.cons_cell->first->value.number->value;
+
     operands = operands->value.cons_cell->rest;
     while (operands->type == Ast_Node_Type_Cons_Cell) {
-        // TODO(Felix): assert type
-        diff -= operands->value.cons_cell->first->value.number->value;
+        assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
+
+        product *= operands->value.cons_cell->first->value.number->value;
+        operands = operands->value.cons_cell->rest;
+    }
+    return create_ast_node_number(product);
+}
+
+Ast_Node* built_in_divide(Ast_Node* operands) {
+    assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
+    double quotient = operands->value.cons_cell->first->value.number->value;
+
+    operands = operands->value.cons_cell->rest;
+    while (operands->type == Ast_Node_Type_Cons_Cell) {
+        assert_type(operands->value.cons_cell->first, Ast_Node_Type_Number);
+
+        quotient /= operands->value.cons_cell->first->value.number->value;
         operands = operands->value.cons_cell->rest;
     }
-    return create_ast_node_number(diff);
+    return create_ast_node_number(quotient);
 }
diff --git a/env.c b/env.c
index bdb9ab5..c829411 100644
--- a/env.c
+++ b/env.c
@@ -54,5 +54,9 @@ Ast_Node* lookup_symbol(Symbol* sym, Environment* env) {
         }
     }
 
+    char* message;
+    asprintf(&message, "Symbol not defined: %s\n", sym->identifier);
+    panic(message);
+
     return NULL;
 }
diff --git a/error.c b/error.c
new file mode 100644
index 0000000..b3d49e3
--- /dev/null
+++ b/error.c
@@ -0,0 +1,6 @@
+typedef struct {
+    char*     message;
+    Ast_Node* location;
+} Error;
+
+Error* error;
diff --git a/eval.c b/eval.c
index a74f492..4b3f5d9 100644
--- a/eval.c
+++ b/eval.c
@@ -5,19 +5,28 @@
 /* } */
 
 Ast_Node* eval_expr(Ast_Node* node, Environment* env);
+int is_truthy (Ast_Node* expression, Environment* env);
 
 void eval_operands(Ast_Node* operands, Environment* env) {
-    while (1) {
+    while (!error) {
         if (operands->type == Ast_Node_Type_Cons_Cell) {
             operands->value.cons_cell->first = eval_expr(operands->value.cons_cell->first, env);
             operands = operands->value.cons_cell->rest;
         } else {
-            break;
+            return;
         }
     }
+    // if we reach here we got an error
+    log_error("An error occurred while evaluating operands to a function:");
 }
 
 Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
+#define report_error(_message)                   \
+    error = new(Error);                          \
+    error->message  = _message;                  \
+    error->location = node;                      \
+    return NULL
+
     Ast_Node* ret = new(Ast_Node);
     switch (node->type) {
     case Ast_Node_Type_Nil:
@@ -44,8 +53,32 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
             } else if (string_equal("-", operator_name)) {
                 eval_operands(operands, env);
                 return built_in_substract(operands);
+            } else if (string_equal("*", operator_name)) {
+                eval_operands(operands, env);
+                return built_in_multiply(operands);
+            } else if (string_equal("/", operator_name)) {
+                eval_operands(operands, env);
+                return built_in_divide(operands);
+            } else if (string_equal("if", operator_name)) {
+                if (Ast_Node_Type_Cons_Cell != operands->value.cons_cell->rest->type                        ||
+                    Ast_Node_Type_Cons_Cell != operands->value.cons_cell->rest->value.cons_cell->rest->type ||
+                    Ast_Node_Type_Nil       != operands->value.cons_cell->rest->value.cons_cell->rest->value.cons_cell->rest->type)
+                {
+                    report_error("Ill formed if statement");
+                }
+
+                Ast_Node* then_part = operands->value.cons_cell->rest;
+                Ast_Node* else_part = then_part->value.cons_cell->rest;
+
+                if (is_truthy(operands->value.cons_cell->first, env))
+                    return eval_expr(then_part->value.cons_cell->first, env);
+                else
+                    return eval_expr(else_part->value.cons_cell->first, env);
+            } else if (string_equal("not", operator_name)) {
+            } else if (string_equal("and", operator_name)) {
+            } else if (string_equal("or", operator_name)) {
             } else {
-                log_message(Log_Level_Critical, "The operator is not yet implemented\n");
+                report_error("The operator is not yet implemented");
             }
         }
 
@@ -55,6 +88,16 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
     }
 
     default:
-        return 0;
+        report_error("Unknown error");
+    }
+#undef report_error
+}
+
+int is_truthy (Ast_Node* expression, Environment* env) {
+    Ast_Node* result = eval_expr(expression, env);
+    switch (result->type) {
+    case Ast_Node_Type_Nil:    return 0;
+    case Ast_Node_Type_Number: return result->value.number->value != 0;
+    default:                   return 1;
     }
 }
diff --git a/helpers.c b/helpers.c
index 1bf6a28..5fb5d4b 100644
--- a/helpers.c
+++ b/helpers.c
@@ -1,3 +1,71 @@
+#define new(type) (type*)malloc(sizeof(type))
+
 int string_equal(char* a, char* b) {
     return !strcmp(a, b);
 }
+
+// asprintf implementation
+int _vscprintf_so(const char * format, va_list pargs) {
+    int retval;
+    va_list argcopy;
+    va_copy(argcopy, pargs);
+    retval = vsnprintf(NULL, 0, format, argcopy);
+    va_end(argcopy);
+    return retval;
+}
+
+int vasprintf(char **strp, const char *fmt, va_list ap) {
+    int len = _vscprintf_so(fmt, ap);
+    if (len == -1) return -1;
+    char *str = malloc((size_t) len + 1);
+    if (!str) return -1;
+    int r = vsnprintf(str, len + 1, fmt, ap); /* "secure" version of vsprintf */
+    if (r == -1) return free(str), -1;
+    *strp = str;
+    return r;
+}
+
+int asprintf(char *strp[], const char *fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    int r = vasprintf(strp, fmt, ap);
+    va_end(ap);
+    return r;
+}
+// asprintf implementation end
+
+char* read_entire_file (char* filename) {
+    char *fileContent = NULL;
+    FILE *fp = fopen(filename, "r");
+    if (fp != NULL) {
+        /* Go to the end of the file. */
+        if (fseek(fp, 0L, SEEK_END) == 0) {
+            /* Get the size of the file. */
+            long bufsize = ftell(fp);
+            if (bufsize == -1) {
+                fputs("Empty file", stderr);
+                goto closeFile;
+            }
+
+            /* Go back to the start of the file. */
+            if (fseek(fp, 0L, SEEK_SET) != 0) {
+                fputs("Error reading file", stderr);
+                goto closeFile;
+            }
+
+            /* Allocate our buffer to that size. */
+            fileContent = malloc(sizeof(char) * (bufsize));
+
+            /* Read the entire file into memory. */
+            size_t newLen = fread(fileContent, sizeof(char), bufsize, fp);
+            if ( ferror( fp ) != 0 ) {
+                fputs("Error reading file", stderr);
+            }
+        }
+      closeFile:
+        fclose(fp);
+    }
+
+    return fileContent;
+    /* Don't forget to call free() later! */
+}
diff --git a/io.c b/io.c
index 5dd8454..6410d62 100644
--- a/io.c
+++ b/io.c
@@ -1,55 +1,60 @@
 typedef enum {
+    Log_Level_None,
+
     Log_Level_Critical,
     Log_Level_Warning,
     Log_Level_Info,
     Log_Level_Debug,
 } Log_Level;
 
+Log_Level log_level = Log_Level_Debug;
+
 void log_message(Log_Level type, char* message) {
+    if (type > log_level)
+        return;
+
     char* prefix;
     switch (type) {
-    case Log_Level_Critical: prefix = "CRIT"; break;
-    case Log_Level_Warning:  prefix = "WARN"; break;
-    case Log_Level_Info:     prefix = "INFO"; break;
-    case Log_Level_Debug:    prefix = "DBUG"; break;
+    case Log_Level_Critical: prefix = "CRITICAL"; break;
+    case Log_Level_Warning:  prefix = "WARNING";  break;
+    case Log_Level_Info:     prefix = "INFO";     break;
+    case Log_Level_Debug:    prefix = "DEBUG";    break;
+    case Log_Level_None:     return;
     }
-    printf("%s: %s",prefix, message);
+    printf("%s: %s\n",prefix, message);
 }
 
-char* read_entire_file (char* filename) {
-    char *fileContent = NULL;
-    FILE *fp = fopen(filename, "r");
-    if (fp != NULL) {
-        /* Go to the end of the file. */
-        if (fseek(fp, 0L, SEEK_END) == 0) {
-            /* Get the size of the file. */
-            long bufsize = ftell(fp);
-            if (bufsize == -1) {
-                fputs("Empty file", stderr);
-                goto closeFile;
-            }
+void log_error(char* message) {
+    log_message(Log_Level_Critical, message);
+    log_message(Log_Level_Critical, error->message);
+}
 
-            /* Go back to the start of the file. */
-            if (fseek(fp, 0L, SEEK_SET) != 0) {
-                fputs("Error reading file", stderr);
-                goto closeFile;
-            }
 
-            /* Allocate our buffer to that size. */
-            fileContent = malloc(sizeof(char) * (bufsize));
+void panic(char* message) {
+    log_message(Log_Level_Critical, message);
+    exit(1);
+}
 
-            /* Read the entire file into memory. */
-            size_t newLen = fread(fileContent, sizeof(char), bufsize, fp);
-            if ( ferror( fp ) != 0 ) {
-                fputs("Error reading file", stderr);
-            }
-        }
-      closeFile:
-        fclose(fp);
-    }
 
-    return fileContent;
-    /* Don't forget to call free() later! */
+char* Ast_Node_Type_to_string (Ast_Node_Type type) {
+    switch (type) {
+    case(Ast_Node_Type_Nil):
+        return "nil";
+    case(Ast_Node_Type_Number):
+        return "number";
+    case(Ast_Node_Type_String):
+        return "string";
+    case(Ast_Node_Type_Symbol):
+        return "symbol";
+    case(Ast_Node_Type_Keyword):
+        return "keyword";
+    case(Ast_Node_Type_Function):
+        return "function";
+    case(Ast_Node_Type_Built_In_Function):
+        return "built-in function";
+    case(Ast_Node_Type_Cons_Cell):
+        return "pair";
+    }
 }
 
 void print(Ast_Node* node) {
@@ -66,6 +71,9 @@ void print(Ast_Node* node) {
     case (Ast_Node_Type_Symbol): {
         printf("%s", node->value.symbol->identifier);
     } break;
+    case (Ast_Node_Type_Keyword): {
+        printf(":%s", node->value.keyword->identifier);
+    } break;
     case (Ast_Node_Type_Function): {
         printf("[lambda]");
     } break;
@@ -73,18 +81,22 @@ void print(Ast_Node* node) {
         printf("[built-in-function %s]", node->value.built_in_function->identifier);
     } break;
     case (Ast_Node_Type_Cons_Cell): {
+        Ast_Node* head = node;
         printf("(");
-        if (node->value.cons_cell->first)
-            print(node->value.cons_cell->first);
-        else
-            printf("nil");
 
-        printf(" . ");
+        while (1) {
+            print(head->value.cons_cell->first);
+            head = head->value.cons_cell->rest;
+            if (head->type != Ast_Node_Type_Cons_Cell)
+                break;
+            printf(" ");
+        }
+
+        if (head->type != Ast_Node_Type_Nil) {
+            printf(" . ");
+            print(head);
+        }
 
-        if (node->value.cons_cell->rest)
-            print(node->value.cons_cell->rest);
-        else
-            printf("nil");
         printf(")");
     } break;
     }
diff --git a/main.c b/main.c
index 366950a..65f8f37 100644
--- a/main.c
+++ b/main.c
@@ -1,55 +1,19 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
-
-#define new(type) (type*)malloc(sizeof(type))
+#include <stdarg.h> /* needed for va_list */
+#include <math.h>
 
 #include "./helpers.c"
 #include "./ast.c"
+#include "./error.c"
 #include "./io.c"
+#include "./assert.c"
 #include "./parse.c"
 #include "./built_ins.c"
 #include "./env.c"
 #include "./eval.c"
-
-void test() {
-    // (+ 4 10)
-    Ast_Node* nil  = create_ast_node_nil();
-    Ast_Node* ten  = create_ast_node_number(10);
-    Ast_Node* four = create_ast_node_number(4);
-    Ast_Node* plus = create_ast_node_symbol("+");
-    Ast_Node* minus = create_ast_node_symbol("-");
-
-    Ast_Node* form1 = create_ast_node_pair(
-        plus,
-        create_ast_node_pair(
-            ten,
-            create_ast_node_pair(
-                four,
-                nil)));
-
-    Ast_Node* form2 = create_ast_node_pair(
-        plus,
-        create_ast_node_pair(
-            four,
-            create_ast_node_pair(
-                four,
-                nil)));
-
-    Ast_Node* root = create_ast_node_pair(
-        minus,
-        create_ast_node_pair(
-            form1,
-            create_ast_node_pair(
-                form2,
-                nil)));
-
-    print(root);
-    printf("\n");
-    Ast_Node* result = eval_expr(root, new(Environment));
-    print(result);
-    printf("\n");
-}
+#include "./testing.c"
 
 int main (int argc, char *argv[]) {
     /* if (argc > 1) { */
@@ -61,7 +25,7 @@ int main (int argc, char *argv[]) {
     /*     printf("No source provided.\n"); */
     /* } */
 
-    /* test(); */
-    print(parse_expression("(+ 4 7)"));
+    run_all_tests();
+
     return 0;
 }
diff --git a/parse.c b/parse.c
index ebb9b5b..f1db90d 100644
--- a/parse.c
+++ b/parse.c
@@ -8,5 +8,5 @@ Ast_Node* parse_expression(char* expr) {
     // initial state is code
     Parser_State current_parser_state = Parser_State_Code;
 
-    
+    return NULL;
 }
diff --git a/testing.c b/testing.c
new file mode 100644
index 0000000..9942060
--- /dev/null
+++ b/testing.c
@@ -0,0 +1,259 @@
+#define console_normal  "\x1B[0m"
+#define console_red     "\x1B[31m"
+#define console_green   "\x1B[32m"
+
+#define epsilon 2.2204460492503131E-16
+
+#define testresult int
+#define pass 1
+#define fail 0
+
+#define print_assert_equal_fail(variable, value, type, format)  \
+    printf("\n\tAssertion failed for '" #variable "'"           \
+           "\n\t  in %s:%d"                                     \
+           "\n\texpected: " format                              \
+           "\n\tgot:      " format "\n",                        \
+           __FILE__, __LINE__, (type)value, (type)variable)
+
+#define print_assert_not_equal_fail(variable, value, type, format)      \
+    printf("\n\tAssertion failed for '" #variable "'"                 \
+           "\n\t  in %s:%d"                                             \
+           "\n\texpected not: " format                                  \
+           "\n\tgot anyways:  " format "\n",                            \
+           __FILE__, __LINE__, (type)value, (type)variable)
+
+#define assert_no_error(error)                                          \
+    if (error) {                                                        \
+        print_assert_not_equal_fail(error, 0, int, "%d");               \
+        printf("\t%s%s%s\n", console_red, error->message,console_normal); \
+        return fail;                                                    \
+    }                                                                   \
+
+#define assert_equal_int(variable, value)                               \
+    if (variable != value) {                                            \
+        print_assert_equal_fail(variable, value, int, "%d");            \
+        return fail;                                                    \
+    }
+
+#define assert_not_equal_int(variable, value)                           \
+    if (variable == value) {                                            \
+        print_assert_not_equal_fail(variable, value, int, "%d");        \
+        return fail;                                                    \
+    }
+
+#define assert_equal_double(variable, value)                            \
+    if (fabs((double)variable - (double)value) > epsilon) {             \
+        print_assert_equal_fail(variable, value, double, "%f");         \
+        return fail;                                                    \
+    }
+
+#define assert_not_equal_double(variable, value)                        \
+    if (fabs((double)variable - (double)value) <= epsilon) {            \
+        print_assert_not_equal_fail(variable, value, double, "%f");     \
+        return fail;                                                    \
+    }
+
+#define assert_null(variable)                   \
+    assert_equal_int(variable, NULL)
+
+#define assert_not_null(variable)               \
+    assert_not_equal_int(variable, NULL)
+
+
+#define invoke_test(name)                                       \
+    printf("" #name ":");                                       \
+    for(int i = 0; i < 45 - strlen(#name); ++i)                 \
+        printf(" ");                                    \
+    if (name() == pass)                                         \
+        printf("%spassed%s\n", console_green, console_normal);  \
+    else {                                                      \
+        printf("%sfailed%s\n", console_red, console_normal);    \
+        if(error) {                                             \
+            log_error("Error while running test" #name);        \
+            free(error);                                        \
+            error = NULL;                                       \
+        }                                                       \
+    }                                                           \
+
+
+testresult test_built_in_add() {
+    Ast_Node* plus = create_ast_node_symbol("+");
+    Ast_Node* ten  = create_ast_node_number(10);
+    Ast_Node* four = create_ast_node_number(4);
+    Ast_Node* nil  = create_ast_node_nil();
+    Ast_Node* form = create_ast_node_pair(
+        plus,
+        create_ast_node_pair(
+            ten,
+            create_ast_node_pair(
+                four,
+                nil)));
+
+    Ast_Node* result = eval_expr(form, new(Environment));
+
+    assert_no_error(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 14);
+
+    return pass;
+}
+
+testresult test_built_in_substract() {
+    Ast_Node* minus = create_ast_node_symbol("-");
+    Ast_Node* ten   = create_ast_node_number(10);
+    Ast_Node* four  = create_ast_node_number(4);
+    Ast_Node* nil   = create_ast_node_nil();
+    Ast_Node* form  = create_ast_node_pair(
+        minus,
+        create_ast_node_pair(
+            ten,
+            create_ast_node_pair(
+                four,
+                nil)));
+
+    Ast_Node* result = eval_expr(form, new(Environment));
+
+    assert_no_error(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 6);
+
+    return pass;
+}
+
+
+testresult test_built_in_multiply() {
+    Ast_Node* times = create_ast_node_symbol("*");
+    Ast_Node* ten  = create_ast_node_number(10);
+    Ast_Node* four = create_ast_node_number(4);
+    Ast_Node* nil  = create_ast_node_nil();
+    Ast_Node* form = create_ast_node_pair(
+        times,
+        create_ast_node_pair(
+            ten,
+            create_ast_node_pair(
+                four,
+                nil)));
+
+    Ast_Node* result = eval_expr(form, new(Environment));
+
+    assert_no_error(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 40);
+
+    return pass;
+}
+
+
+testresult test_built_in_divide() {
+    Ast_Node* over = create_ast_node_symbol("/");
+    Ast_Node* ten  = create_ast_node_number(20);
+    Ast_Node* four = create_ast_node_number(4);
+    Ast_Node* nil  = create_ast_node_nil();
+    Ast_Node* form = create_ast_node_pair(
+        over,
+        create_ast_node_pair(
+            ten,
+            create_ast_node_pair(
+                four,
+                nil)));
+
+    Ast_Node* result = eval_expr(form, new(Environment));
+
+    assert_null(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 5);
+
+    return pass;
+}
+
+
+testresult test_built_in_if() {
+    Ast_Node* _if    = create_ast_node_symbol("if");
+    Ast_Node* cond   = create_ast_node_number(1);
+    Ast_Node* four   = create_ast_node_number(4);
+    Ast_Node* five   = create_ast_node_number(5);
+    Ast_Node* nil    = create_ast_node_nil();
+    Ast_Node* form   = create_ast_node_pair(
+        _if,
+        create_ast_node_pair(
+            cond,
+            create_ast_node_pair(
+                four,
+                create_ast_node_pair(
+                    five,
+                    nil))));
+    Ast_Node* result;
+
+    // test *then* case
+    result = eval_expr(form, new(Environment));
+
+    assert_no_error(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 4);
+
+    // test *else* case
+    cond->value.number->value = 0;
+    result = eval_expr(form, new(Environment));
+
+    assert_null(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 5);
+
+    return pass;
+}
+
+testresult test_built_in_and() {
+    Ast_Node* _and  = create_ast_node_symbol("and");
+    Ast_Node* cond1 = create_ast_node_number(1);
+    Ast_Node* cond2 = create_ast_node_string("asd");
+    Ast_Node* cond3 = create_ast_node_number(4);
+    Ast_Node* nil   = create_ast_node_nil();
+    Ast_Node* form  = create_ast_node_pair(
+        _and,
+        create_ast_node_pair(
+            cond1,
+            create_ast_node_pair(
+                cond2,
+                create_ast_node_pair(
+                    cond3,
+                    nil))));
+    Ast_Node* result;
+
+    // a true case
+    result = eval_expr(form, new(Environment));
+
+    assert_no_error(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 1);
+
+    // a false case
+    cond1->value.number->value = 0;
+    result = eval_expr(form, new(Environment));
+
+    assert_no_error(error);
+    assert_not_null(result);
+    assert_equal_int(result->type, Ast_Node_Type_Number);
+    assert_equal_double(result->value.number->value, 0);
+
+    return pass;
+}
+
+
+void run_all_tests() {
+    log_level = Log_Level_None;
+
+    invoke_test(test_built_in_add);
+    invoke_test(test_built_in_substract);
+    invoke_test(test_built_in_multiply);
+    invoke_test(test_built_in_divide);
+    invoke_test(test_built_in_if);
+    invoke_test(test_built_in_and);
+
+}
diff --git a/todo.org b/todo.org
index f29294b..177e31a 100644
--- a/todo.org
+++ b/todo.org
@@ -1,35 +1,41 @@
 * Build-in forms
 
- - +
- - -
- - *
- - /
- - >
- - <
- - =
+** DONE +
+   CLOSED: [2018-09-18 Di 12:14]
+** DONE -
+   CLOSED: [2018-09-18 Di 12:14]
+** DONE *
+   CLOSED: [2018-09-18 Di 12:14]
+** DONE /
+   CLOSED: [2018-09-18 Di 12:14]
+** TODO >
+** TODO <
+** TODO =
+
+** DONE if
+   CLOSED: [2018-09-18 Di 12:14]
+** TODO and
+** TODO or
+** TODO not
+
+** TODO first  (car)
+** TODO rest   (cdr)
+** TODO pair   (cons)
+
+** TODO load   (import)
+** TODO define
+
+** TODO lambda
+** TODO progn
+
+** TODO eval
+** TODO quote
+
+** TODO print
+** TODO read
+
+** TODO help
 
- - if
- - and
- - or
- - not
-
- - first  (car)
- - rest   (cdr)
- - pair   (cons)
-
- - load   (import)
- - define
-
- - lambda
- - progn
-
- - eval
- - quote
-
- - print
- - read
-
- - help
 * Types
  - Symbol
  - Number