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

theorem
  for D be non empty set, F be PartFunc of D,REAL, d be Element of D st
  d in dom F holds Sum(F,{d}) = F.d
proof
  let D be non empty set, F be PartFunc of D,REAL, d be Element of D;
   reconsider Fd = F.d as Element of REAL by XREAL_0:def 1;
  assume
  d in dom F;
  hence Sum(F,{d}) = Sum <*Fd*> by Th69
    .= F.d by FINSOP_1:11;
end;
