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

theorem Th49: :: Lemma 4
  \delta (x _1 + x _3, x) = x _0
proof
  x _0 = Expand (x _4, x _0) by Th36
    .= \delta (x _4 + x _0, \delta (x _4, \delta (x _3, x))) by Th48
    .= \delta (x _3 + x _1, \delta (x _4, \delta (x _3, x))) by Th42
    .= \delta (x _3 + x _1, Expand (x _3, x)) by Th44
    .= \delta (x _3 + x _1, x) by Th36;
  hence thesis;
end;
