## TPTP Problem File: ROB003-1.p

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```%--------------------------------------------------------------------------
% File     : ROB003-1 : TPTP v7.5.0. Released v1.0.0.
% Domain   : Robbins Algebra
% Problem  : X + c=c => Boolean
% Version  : [Win90] (equality) axioms.
% English  : If there exists c such that X+c=c, then the algebra
%            is Boolean.

% Refs     : [HMT71] Henkin et al. (1971), Cylindrical Algebras
%          : [Win90] Winker (1990), Robbins Algebra: Conditions that make a
%          : [LW92]  Lusk & Wos (1992), Benchmark Problems in Which Equalit
% Source   : [OTTER]
% Names    : Lemma 2.2 [Win90]
%          : RA1 [LW92]
%          : robbins.in [OTTER]

% Status   : Unsatisfiable
% Rating   : 0.25 v7.5.0, 0.12 v7.4.0, 0.26 v7.3.0, 0.16 v7.1.0, 0.06 v7.0.0, 0.11 v6.4.0, 0.21 v6.3.0, 0.12 v6.2.0, 0.07 v6.1.0, 0.19 v6.0.0, 0.33 v5.5.0, 0.32 v5.4.0, 0.13 v5.3.0, 0.08 v5.2.0, 0.14 v5.1.0, 0.20 v5.0.0, 0.14 v4.1.0, 0.09 v4.0.1, 0.07 v4.0.0, 0.08 v3.7.0, 0.00 v2.2.1, 0.22 v2.2.0, 0.29 v2.1.0, 0.25 v2.0.0
% Syntax   : Number of clauses     :    5 (   0 non-Horn;   5 unit;   1 RR)
%            Number of atoms       :    5 (   5 equality)
%            Maximal clause size   :    1 (   1 average)
%            Number of predicates  :    1 (   0 propositional; 2-2 arity)
%            Number of functors    :    5 (   3 constant; 0-2 arity)
%            Number of variables   :    8 (   1 singleton)
%            Maximal term depth    :    6 (   3 average)
% SPC      : CNF_UNS_RFO_PEQ_UEQ

% Comments : Commutativity, associativity, and Huntington's axiom
%            axiomatize Boolean algebra.
%          : In Overbeek's version, the hypothesis is slightly different :
%            ...an element c such that c+c=c, then... Mail from McCune says
%            that this is a simpler problem.
%--------------------------------------------------------------------------
%----Include axioms for Robbins algebra
include('Axioms/ROB001-0.ax').
%--------------------------------------------------------------------------
cnf(there_exists_a_constant,hypothesis,