theorem Th3:
  for n be non zero Element of NAT,
      f be PartFunc of REAL,REAL n,
      h be PartFunc of REAL,REAL-NS n,
      x be Real st h=f
holds diff(f,x) = diff(h,x)
proof
  let n be non zero Element of NAT,
      f be PartFunc of REAL,REAL n,
      h be PartFunc of REAL,REAL-NS n,
      x be Real;
  ex g be PartFunc of REAL,REAL-NS n st f=g & diff(f,x) = diff(g,x) by Def2;
  hence thesis;
end;
