tcfail103.hs:15:23:
    Couldn't match type ‛t’ with ‛s’
      ‛t’ is a rigid type variable bound by
          the type signature for f :: ST t Int at tcfail103.hs:10:5
      ‛s’ is a rigid type variable bound by
          the type signature for g :: ST s Int at tcfail103.hs:13:14
    Expected type: STRef s Int
      Actual type: STRef t Int
    Relevant bindings include
      g :: ST s Int (bound at tcfail103.hs:15:9)
      v :: STRef t Int (bound at tcfail103.hs:12:5)
      f :: ST t Int (bound at tcfail103.hs:11:1)
    In the first argument of ‛readSTRef’, namely ‛v’
    In the expression: readSTRef v