
theorem
  for L being properly_defined Boolean well-complemented Lattice-like
  non empty ShefferOrthoLattStr holds L is satisfying_Sheffer_1
proof
  let L be properly_defined Boolean well-complemented Lattice-like non empty
  ShefferOrthoLattStr;
  let x be Element of L;
  x`` = x by ROBBINS1:3;
  then (x | x)` = x by Def12;
  hence thesis by Def12;
end;
