%  Challenge problem from Peter Andrews (1979)
%
%  Although this problem is more easily solved by direct simplification
%  of the quantified formula (Champeaux, J. ACM 1986 and SIGART Newsletter), 
%  it makes a good test problem for resolution theorem provers.  Otter
%  can do this problem, because it translates equivalences in two ways,
%  depending on the context, producing only 128 clauses.  (Also, FormEd
%  can prove it by direct simplification.)
%

set(binary_res).
set(process_input).
clear(print_kept).
clear(print_back_sub).

assign(max_mem, 1500).      % 1.5 Megabytes

formula_list(sos).

% Andrews challenge problem

-(
   (
     (exists x all y (p(x) <-> p(y)))
     <->
     ((exists u q(u)) <-> (all v p(v)))
   )
   <->
   (
     (exists w all z (q(z) <-> q(w)))
     <->
     ((exists x1 p(x1)) <-> (all x2 q(x2)))
   )
 ).

end_of_list.

