We prove that anon is an otter.
Let O be the set that contains all otters.
Further, let D be the set of all mustelids, and M be the set of all mammals.
We define
O := { x | x ∈ D ∧ x hunts fish },
D := { x | x ∈ M ∧ x is long },
M := { x | x is hairy }.
Trivially, O ⊆ D ⊆ M.
Therefore, we must show that
1) Anon is hairy,
2) Anon is long, and
3) Anon hunts fish.
For 1), if anon didn't have hair he would have scales or feathers. No being with scales or feathers possesses fingers, therefore they cannot post on /otter/.
Anon can post on /otter/ ⇒anon has fingers ⇒anon is hairy.
Therefore, anon ∈ M.
For 2), assume anon is not long. If anon is not long, then he would not be long enough to reach the keyboard to post on /otter/. However, anon posts on /otter/, therefore anon must be long enough to post on /otter/. Hence, anon is long.
For 3), we know that anon eats fish. In order to be eaten, fish must be hunted. Anon can eat the fish iff it has been hunted. Therefore if anon didn't hunt the fish, he could not eat it, which is a contradiction. Hence, anon hunts fish.
Anon is long, hairy, and hunts fish. Therefore, anon is an otter. QED
