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

theorem Th36:
  \delta ((x + y), (\delta (x, y))) = y
proof
  thus \delta ((x + y), (\delta (x, y))) = -(-(x + y) + -(-x + y))
    .= y by Def5;
end;
