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

theorem Th44:
  x _3 + x = x _4
proof
  thus x _3 + x = x _0 + (Double x + x + x) by LATTICES:def 5
    .= x _4 by LATTICES:def 5;
end;
