
theorem
  for L being properly_defined Boolean well-complemented Lattice-like
  non empty ShefferOrthoLattStr holds L is satisfying_Sheffer_2
proof
  let L be properly_defined Boolean well-complemented Lattice-like non empty
  ShefferOrthoLattStr;
  let x, y be Element of L;
  x` + Bot L = x` by ROBBINS1:13;
  then x` + (y`` *' y`) = x` by ROBBINS1:15;
  then x` + (y` + y)` = x` by Th1;
  then x | (y` + y) = x` by Def12;
  then x | (y` + y``) = x` by ROBBINS1:3;
  then x | (y | (y`)) = x` by Def12;
  then x | (y | (y | y)) = x` by Def12
    .= x | x by Def12;
  hence thesis;
end;
