## TPTP Problem File: ROB007-4.p

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```%--------------------------------------------------------------------------
% File     : ROB007-4 : TPTP v7.5.0. Bugfixed v3.1.0.
% Domain   : Robbins Algebra
% Problem  : Absorbed within negation element => Exists idempotent element
% Version  : [Win90] (equality) axioms : Augmented.
% English  : If there exist a, b such that -(a+b) = -b, then the algebra
%            is Boolean.

% Refs     : [HMT71] Henkin et al. (1971), Cylindrical Algebras
%          : [Win90] Winker (1990), Robbins Algebra: Conditions that make a
% Source   : [Win90]
% Names    : Theorem 1.2 [Win90]

% Status   : Unknown
% Rating   : 1.00 v3.1.0
% Syntax   : Number of clauses     :   12 (   0 non-Horn;   8 unit;   7 RR)
%            Number of atoms       :   16 (  12 equality)
%            Maximal clause size   :    2 (   1 average)
%            Number of predicates  :    2 (   0 propositional; 1-2 arity)
%            Number of functors    :    7 (   3 constant; 0-2 arity)
%            Number of variables   :   18 (   1 singleton)
%            Maximal term depth    :    7 (   3 average)
% SPC      : CNF_UNK_RFO_SEQ_HRN

% Comments : Commutativity, associativity, and Huntington's axiom
%            axiomatize Boolean algebra.
%          : The two extra lemmas are suggested by [Win90].
% Bugfixes : v3.1.0 - Removed extra negated_conjecture clauses.
%--------------------------------------------------------------------------
%----Include axioms for Robbins algebra
include('Axioms/ROB001-0.ax').
%----Include axioms for numbers in Robbins algebras
include('Axioms/ROB001-1.ax').
%--------------------------------------------------------------------------
cnf(absorbtion,axiom,

cnf(corollary_3_9_1,axiom,