Totally new implementation of comtabs. Now DEFINE-KEY and

Totally new implementation of comtabs.  Now DEFINE-KEY and
DEFINE-PREFIX-KEY handle lower-case letters specially: if the
corresponding upper-case letter is undefined, it is converted into an
alias for the lower-case letter.  Additional changes:
DEFINE-DEFAULT-KEY eliminated (it was unused); comtabs support aliases
much like Emacs does; internal structure of comtab varies depending on
the contents of the comtab.