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Fix Bug #612; COPYDEF of procedure defined with DEFINE now has the correct title line.

Change mustDerefStringnode to mustGcrefStringnode in numberp().
A ref and a deref are not necessary. A simple gcref suffices.
Unfortunately, the flag is still needed because numberp's argument may not have any references.

Add a missing line break

Refactoring: Change lbutlast to use CAppendableList.
This reduces code duplication.

Rework char_arg to take the argument instead of an argument list.
Also, rename it to validate_char_argument since it follows the new conventions of the validate* functions.

Refactoring: Change the signature of char_arg() to return a wchar_t instead of NODE*.
This is what all of the callers want, anyway.

This change makes it clear that char_arg() is responsible for freeing whatever it allocates.

Rework char_arg() so that it doesn't modify the input list.
This is part of a wider effort to simplify the code base by removing side effects.

Add additional tests for ASCII to prepare for refactoring of char_arg().

Fix the way the empty string is expressed in some RUNRECOVERABLEERRORTEST tests.
It's currently impossible to put an empty string into a list literal.
What looked like the empty string ended up being ignored.

This change replaces the list literal with a call to LIST, which can create a list that contains the empty string.

Refactoring: rename lbfs to lbutfirsts for consistency with how other logofuncs are named.
Most other primitives are implemented by functions that are named as "l" concatenated to the primitive's full name, not an abbreviation.