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

theorem :: Winker Second Condition
  ex y, z st -(y + z) = -z
proof
  set x = the Element of G;
  take y = x _1, z = x _3;
  -(y + z) = \delta (\beta x, x) by Th53
    .= -z by Th52;
  hence thesis;
end;
