Illegal instance declaration, flexible instances?

test.hs

{-# LANGUAGE MultiParamTypeClasses #-}
class Collection c a where
    insert :: a -> c a -> c a

instance Collection [] a where
    insert a b = (a:b)

{-
    Illegal instance declaration for `Collection [] a'
      (All instance types must be of the form (T a1 ... an)
       where a1 ... an are *distinct type variables*,
       and each type variable appears at most once in the instance head.
       Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Collection [] a'
 -}
-- http://www.haskell.org/haskellwiki/List_instance