How to define Sage function with Optional arguments?

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To do optional arguments in Python (and hence, Sage), the idiom is something like

def f(a=2,b=3,*args):
    <function definition>
    first_optional_argument = args[0]
    <etc>

where args then becomes a list of the optional arguments. See much more detailed stuff:

• A Stackoverflow question
• A blog post
• A Sage-specific programming tutorial (search for *args)