Add nelss to iset.mm

Copied without change from set.mm.

Author: jkingdon

iset.mm

@@ -27453,6 +27453,11 @@ practical reasons (to avoid having to prove sethood of ` A ` in every use
       IAJBCKPQALABCMN $.
   $}
 
+  $( Demonstrate by witnesses that two classes lack a subclass relation.
+     (Contributed by Stefan O'Rear, 5-Feb-2015.) $)
+  nelss $p |- ( ( A e. B /\ -. A e. C ) -> -. B C_ C ) $=
+    ( wcel wss ssel com12 con3dimp ) ABDZBCEZACDZJIKBCAFGH $.
+
   ${
     $d x A $.  $d x B $.
     $( Quantification restricted to a subclass.  (Contributed by NM,