Generic functions: Specializers like `or' or `memq'?

[Michael Heerdegen]

Hello,

I come back to this question again and again, so it's time to just ask:

Often I want to use a certain method implementation for several very
similar cases and want to avoid code duplication, but I don't know how
to express this using the existing method specializers.

For example, I want the `head' spec to match one of several symbols who
are synonymous.  Or instead of (eql SYM) I would rather want
(memq (SYMS...)).  Or I want to combine several SPECS using `or' because
the implementation is the same.

Am I'm missing something or - this is not possible currently, right?
Would it be possible to implement such stuff then?

TIA,
Michael.