:set -XExistentialQuantification -XDeriveDataTypeable -XGeneralizedNewtypeDeriving
:l Test
let a = O (id 'a')
let b = O (id "abc")
:p a
:p b
seq _t1 ()
:p _t1
:show bindings
_t2
-- bogus!! _t2 has been resolved to type Char