theorem Th13:
  len GoB(f) = card rng X_axis(f) & width GoB(f) = card rng Y_axis (f)
proof
  set x = X_axis(f), y = Y_axis(f);
  len Incr(x)=card rng x & len Incr(y)=card rng y by SEQ_4:def 21;
  hence thesis by Def1;
end;
