Tweak abeq2 example to also not elsymdifxor
Tweak abeq2 example to also not elsymdifxor

Theorem abeq2 pointed to theorem cp , but cp is somewhat complex
and may be hard for a newcomer to follow.  Sometimes there are simpler
ways to do the conversion! This commits adds a link to
theorem dfsymdif2 as a simpler example of a conversion.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm