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

theorem Th47: :: Exchange
  \delta (x, -y) = \delta (y, -x)
proof
  thus \delta (x, -y) = -(-x + -y) .= \delta (y, -x);
end;
