implicit parameters

[Dean Herington]

There's apparently something very basic that I don't understand about
implicit parameters.  In the following program, compilation fails if
either of the type declarations is omitted.  What is "implicit" about
implicit parameters, then?

{-# OPTIONS -fglasgow-exts #-}

main = print f1

f1 = let ?f = \ x -> x ++ x in f2

f2 :: (?f :: String -> String) => String
f2 = f3

f3 :: (?f :: String -> String) => String
f3 = ?f "x"