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Refactoring: Change SETSCRUNCH to use flonum_arg instead of numeric_arg.
flonum_arg returns a FLONUM, which is what SETSCRUNCH wants.
It also doesn't have any side effects which change the argument list.

This is part of a larger effort to make the code base simpler by removing unneeded side effects.

Remove some misleading comments that were copied&pasted from other tests.

Add some more bad input tests for SETSCRUNCH, including the use of :erract.

Refactoring: Change setpos_helper() to accept "const FLONUM*" instead of "NODE*".
This adds type safety because NODE* can be any type, but a FLONUM* can only be a FLONUM.
This also clarifies that setpos_helper() doesn't modify its arguments, as NODE* can be modified but "const FLONUM*" can't.

Add tests for SETXYZ to the full test pass.

Fix a copy&paste error in a comment

Add tests for SETXY

Refactoring: Use RUNLOGOPROGRAM instead of duplicating its logic.

Add tests for ZCOR.

Add tests for YCOR to the full test pass.