I imagine this has been asked and answered before, so I apologize in
advance.

Compare these two functions:

# let f x = List.map (fun y -> y) x ;;
val f : 'a list -> 'a list = <fun>

and

# let f = List.map (fun y -> y) ;;
val f : '_a list -> '_a list = <fun>

Why does the second use the '_a type variable instead of 'a?  I thought
that special type variables only had to do with polymorphic references.
The FAQ states that the latter expression can't be generalized, but I don't
really understand why.

Mike
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