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

theorem Th41: :: Lemma 1
  \delta (x _2, x) = x _0
proof
  thus \delta (x _2, x) = \delta (Double x + x _0, \delta (Double x, x _0)) by
Th40
    .= x _0 by Th36;
end;
