For a comparison `` expr1 expr2'', expr1
and expr2 must be lists or symbols.
If expr1 and expr2 are symbols, the list of their atomic symbols ( atoms( expr1) and atoms( expr2) will be used for the comparison instead of the symbols themself.
The comparison test whether the lists do congruate, this means, whether they have an element in common.
The comparison `` expr1 / expr2'' holds iff the
comparison `` expr1
expr2'' does not hold.