#1 and #1# in expressions

For debugging, I placed a print at the top of $is:

(defmspec $is (form)
  (print `(in $is x = ,x))

Now I see

 (%i3) is([a,b]=[c,d]);
 (IN $IS X = (($IS) ((MEQUAL) (#1=(MLIST) $A $B) (#1# $C $D))))

What's the story with #1 and #1#?

Barton