reserve L for satisfying_Sh_1 non empty ShefferStr;
reserve L for satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3
  non empty ShefferStr;
reserve v,q,p,w,z,y,x for Element of L;

theorem Th121:
  for x,y holds x = (x | x) | (y | x)
proof
  let x,y;
  (y | x) | ((y | y) | x) = x by Th92;
  hence thesis by Th120;
end;
