xlib:create-glyph-cursor -- a spec issue

[Mike McMahon <MMcM@STONY-BROOK.SCRC.Symbolics.COM>]

    Date: Wed, 26 Jul 89  10:11:14 EDT
    From: Kerry Kimbrough <Kimbrough@dsg.csc.ti.com>

    I stumbled on a minor lacuna in the CLX spec for create-glyph-cursor.  What is
    supposed to happen when mask-font is specified but mask-char is omitted?  The
    code just wires mask-char=0, which doesn't seem particularly useful. Several
    alternatives are possible.

    1. Signal error
	
    2. Default to source-char

    3. Default to 0

    4. Default to (font-default-char mask-font)

    5. If (eq source-font mask-font), default to (1+ source-char), else signal
       error.

    I suggest #5.  Since the source/mask-char's have to be tightly coupled, it's not
    reasonable that any single value could serve as a default mask-char, so #2 is
    plausible but #1 is more correct.  But #5 also allows a short-hand for a common
    idiom based on a special font structure (like that of the "cursor" font).

5 sounds reasonable, especially if mask-font defaults to source-font.