A function to call to parse the current line's arguments.
It should be called with no parameters, and with point at the position
of the argument that is to be completed.
It must either return nil, or a cons cell of the form:
((ARG...) (BEG-POS...))
The two lists must be identical in length. The first gives the final
value of each command line argument (which need not match the textual
representation of that argument), and BEG-POS gives the beginning
position of each argument, as it is seen by the user. The establishes
a relationship between the fully resolved value of the argument, and
the textual representation of the argument.