Where is the beginning of the code?

I know a little C programming language, and aren’t all programs in C suppose to start with main(); the main function? Where is the full page of code for these snippets of code?

The function definition is part of the interface:

The def plan(self): starts defining the plan function, like you would define the main fuction in a C program. Then our code calls into your code by calling the plan function that you defined.