reserve L for satisfying_Sh_1 non empty ShefferStr;

theorem Th58:
  for x, y, z being Element of L holds x | (y | (y | (z | (x | y)) )) = x | x
proof
  let x, y, z be Element of L;
  x | (y | (x | x)) = x | x by Th12;
  hence thesis by Th57;
end;
