theorem Th54:
  f|Y is constant implies f is_bounded_on Y
proof
  assume f|Y is constant;
  then consider r being VECTOR of V such that
A1: for c st c in Y /\ dom f holds f/.c = r by PARTFUN2:35;
  now
    take p=||.r.||;
    let c;
    assume c in Y /\ dom f;
    hence ||.f/.c.|| <= p by A1;
  end;
  hence thesis;
end;
