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:     Q;
  dnf({Q,D}) ->
      dnf(mv_not(Q), D).
  
  %%----------------------------------------------------------------------------
  %% Internal functions
  
negative.code: %% API
  -export([from_str/2]).
  -export([dnf/1]).
  -export([clauses/1]).
  -export([depth/1]).
  
  %% Record declarations
  -record(and_q,  {l_sub :: q(), r_sub :: q()}).
  