X : Base.T.T3
val lift : ('a1, 'b1) Base__Type_equal.t ‑> ('a2, 'b2) Base__Type_equal.t ‑> ('a3, 'b3) Base__Type_equal.t ‑> (('a1, 'a2, 'a3) X.t, ('b1, 'b2, 'b3) X.t) Base__Type_equal.t