(if (livesNearWater x) then (not (livesInDesert x)))
(if (laysEggsInWater x) then (livesNearWater x))
(if (Amphibian x) then (laysEggsInWater x))
(if (Frog x) then (Amphibian x))


(Frog Fred) concludes (not (livesInDesert Fred))

the conclusion is correct, but the chain of reasoning is invalid.

but not adequate to conclude:

Fred sometimes gets wet.
Fred lays eggs.
Fred is alive.
Fred doesn't live in Sahara.
Fred doesn't live on the surface of the sun.
Fred is not a mongoose or a bird.