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Rework some logic in paren_infix to avoid a redundant assignment.
This was done primarily for clarity; I didn't like that right_arg included left_arg, even if temporarily.

Remove a special case of Minus_Tight from priority().
This does not need to be a special case, since it calcluated the same by the common case.
The priority is already specified in the prims table and specifying it redundantly in priority() violates the DRY principal.

Add/correct some comments in paren_infix

Restructure some logic in gather_args to avoid redundant computation.
This may also make the code easier to read.

Cleanup: Remove code that looks useless.
By inspection, if max is less then zero, then set to zero, it
will immediately be decremented back to a negative value.

Cleanup: remove inparen parameter from gather_some_args.
This hasn't been used since bug #566 was fixed.

Fix a typo in a comment

Rename gather_more_args to argument_is_available and add "const" to the argument.
This emphasizes that the function is a simple predicate with no side-effects.
That is, it doesn't gather anything, it just says whether an argument could be gathered.

Declare gather_some_args as static, since it is not used outside this file.

Add/improve some comments