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

theorem
  for D be non empty set, G1,G2,G3 be Element of PFuncs(D,REAL) holds
  Sum<*G1,G2,G3*> = G1 + G2 + G3
proof
  let D be non empty set, G1,G2,G3 be Element of PFuncs(D,REAL);
  thus Sum<*G1,G2,G3*> = Sum(<*G1,G2*>^<*G3*>) by FINSEQ_1:43
    .= Sum<*G1,G2*> + G3 by Th20
    .= G1 + G2 + G3 by Th23;
end;
