reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th69:
  for L being non empty ShefferStr st L is satisfying_Sh_1 holds L
  is satisfying_Sheffer_3
proof
  let L be non empty ShefferStr;
  assume L is satisfying_Sh_1;
  then
  for x, y, z being Element of L holds (x | (y | z)) | (x | (y | z)) = ((y
  | y) | x) | ((z | z) | x) by Th66;
  hence thesis by SHEFFER1:def 15;
end;
