
theorem
  for X being set, F being PartFunc of X,ExtREAL holds F is nonnegative
  iff for n being object holds 0. <= F.n
proof
  let X be set, F be PartFunc of X,ExtREAL;
  hereby
    assume F is nonnegative; then
A1: rng F is nonnegative;
    let n be object;
    per cases;
    suppose
      n in dom F;
      then F.n in rng F by FUNCT_1:def 3;
      hence 0. <= F.n by A1;
    end;
    suppose
      not n in dom F;
      hence 0. <= F.n by FUNCT_1:def 2;
    end;
  end;
  assume
A2: for n being object holds 0. <= F.n;
  let y be ExtReal;
  assume y in rng F;
  then ex x being object st x in dom F & y = F.x by FUNCT_1:def 3;
  hence thesis by A2;
end;
