# Functions
ParseOne sees if args[0] is present in the given flag set and if so, sets its value and returns the flag along with the remaining (unused) arguments.
# Variables
ErrFlagTerminator indicates the distinguished token "--", which causes the flag package to treat all subsequent arguments as non-flags.
# Structs
A FlagNotDefinedError indicates a flag-like argument that does not correspond to any registered flag in a FlagSet.
A NonFlagError indicates an argument that is not a syntactically-valid flag.