reserve n,m for Nat,
  r,r1,r2,s,t for Real,
  x,y for set;

theorem Th18:
  for D be non empty set holds addpfunc(D) is having_a_unity
proof
  let D be non empty set;
  take [#](D) --> In(0,REAL);
  thus thesis by Th16;
end;
