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 Th87:
  for x,p holds (p | x) | (p | x) = (x | p) | (x | p)
proof
  let x,p;
  (x | x) | (x | x) = x by SHEFFER1:def 13;
  hence thesis by Th86;
end;
