anchor.diff:  prop_no_nested_or() ->
       ?FORALL(X, q(), not nested_or(dnf({X,depth(X)}))).
   
  -nested_or({q,L,R}) ->
  +nested_or({or_q,L,R}) ->
  +    nested_or(L) or nested_or(R);
  +nested_or({and_q,L,R}) ->
       (is_record(L, or_q) or is_record(R, or_q))
           orelse (nested_or(L) or nested_or(R));
  -nested_or({or_q,L,R}) ->
  -    nested_or(L) or nested_or(R);
   nested_or({not_q,Q}) ->
       nested_or(Q);
   nested_or(_) ->
  
positive.code: not_element(#not_q{}) ->
      false.
  
  depth({_,L,R}) ->
      max(depth(L),depth(R))+1;
  depth({not_q,Q}) ->
      depth(Q)+1;
  
negative.code: %% Tests
  %%----------------------------------------------------------------------------
  
  -ifdef(TEST).
  
  proper_test_() ->
      [{atom_to_list(F),
  