Ast_Node* built_in_add(Ast_Node* operands) {
    double sum = 0;
    while (operands->type == Ast_Node_Type_Cons_Cell) {
        // TODO(Felix): assert type
        sum += operands->value.cons_cell->first->value.number->value;
        operands = operands->value.cons_cell->rest;
    }

    return create_ast_node_number(sum);
}

Ast_Node* built_in_substract(Ast_Node* operands) {
    double diff = 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;
        operands = operands->value.cons_cell->rest;
    }
    return create_ast_node_number(diff);
}