let x, y be set ; :: according to TAXONOM2:def 3 :: thesis: ( x in {} & y in {} & not x c= y & not y c= x implies x misses y )
assume that
A1: x in {} and
y in {} ; :: thesis: ( x c= y or y c= x or x misses y )
thus ( x c= y or y c= x or x misses y ) by A1; :: thesis: verum