tcfail198.hs:6:36: error:
    Couldn't match expected type ‘a2’ with actual type ‘a1’
      because type variable ‘a2’ would escape its scope
    This (rigid, skolem) type variable is bound by
      an expression type signature: a2
      at tcfail198.hs:6:36-41
    In the expression: x :: a
    In the second argument of ‘(++)’, namely ‘[x :: a]’
    Relevant bindings include
      xs :: [a1] (bound at tcfail198.hs:6:21)
      x :: a1 (bound at tcfail198.hs:6:19)
      f3 :: [a1] -> [a1] (bound at tcfail198.hs:6:6)