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 Th131:
  for y,x holds ((x | y) | (x | y)) | x = x | y
proof
  let y,x;
  (x | (y | y)) | (x | y) = x by Th109;
  hence thesis by Th121;
end;
