anchor.diff:  
   dnf(Q, 1) ->
       Q;
  -dnf(Q, D) ->    
  +dnf(Q, D) ->
       dnf(mv_and(Q), D-1).
   
   mv_not({T,L,R}) ->
  
positive.code:     {T,mv_and(L),mv_and(R)};
  mv_and(Q) ->
      Q.
  
  %%----------------------------------------------------------------------------
  %% PropErties
  
negative.code:     Q;
  dnf({Q,D}) ->
      dnf(mv_not(Q), D).
  
  clauses(Q) ->
      flatten(and_subs(Q)).
  
  %%----------------------------------------------------------------------------
  %% Internal functions
  