reserve G for Robbins join-associative join-commutative non empty
  ComplLLattStr;
reserve x, y, z, u, v for Element of G;

theorem
  (for z holds --z = z) implies G is Huntington
proof
  assume
A1: for z holds --z = z;
  let x, y;
A2: --(-(-x + -y) + -(x + -y)) = --y by Def5;
  -(-x + -y) + -(x + -y) = --(-(-x + -y) + -(x + -y)) by A1
    .= y by A1,A2;
  hence thesis;
end;
