reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th67:
  for L being non empty ShefferStr st L is satisfying_Sh_1 holds L
  is satisfying_Sheffer_1
proof
  let L be non empty ShefferStr;
  assume L is satisfying_Sh_1;
  then for x being Element of L holds (x | x) | (x | x) = x by Th21;
  hence thesis by SHEFFER1:def 13;
end;
