Okay, so let's see:

All triangles are 3-sided

A -> B

not 3-sided -> not a triangle

~B -> ~A

squares are 4-sided

C -> D

not 4-sided -> not a square

~D -> ~C

A square -> not 3-sided

C -> ~B

a 4-sided object -> not a triangle

D -> ~A

So everytime we see a non 3-sided object (because it is 4-sided) it is reinforcing the idea that it is not a triangle (it is a square)