static <K>void bug(Set<Collection<K>> set,Collection<K> c,K k){
  set.add(c);
  c.add(k);
}

static < K > void bug ( Set < Collection < K > > set , Collection < K > c , K k ) { set . add ( c ) ; c . add ( k ) ; }
bug to mutate object after it has been added to a set or map
