theorem Th51: :: Lemma 6
  \delta (x _1 + x _3, x _0) = x
proof
  thus x = Expand (x _1 + x _2, x) by Th36
    .= \delta (x _1 + (x _2 + x), \delta(x _1 + x _2, x)) by LATTICES:def 5
    .= \delta (x _1 + x _3, \delta(x _1 + x _2, x)) by LATTICES:def 5
    .= \delta (x _1 + x _3, x _0) by Th50;
end;
