All frozen harbours exclude ships.
The harbour is frozen.
... It excludes ships.
Again, take an example of the Modus Tollens—
If rain has fallen, the streets are wet.
The streets are not wet.
... Rain has not fallen.
This is reducible, by formulating the underlying proposition, to Camestres or Baroko of the Second Figure.
All streets rained upon are wet.
The streets are not wet.