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

theorem Th45: :: Lemma 2
  \delta (x _3, x _0) = x
proof
  thus x = Expand (x _2, x) by Th36
    .= \delta (x + x _2, x _0) by Th41
    .= \delta (x _3, x _0) by LATTICES:def 5;
end;
