I still can't wrap my head around the whole polymorphic variant of
Ocaml (although I am making steady progresses). Anyways here is my
latest sore point:

This expression cannot be coerced to type
  O.o Ast.T.expr O.m = O.o Ast.T.expr Mon.m;
it has type O.o Ast_js.T.expr Mon.m but is here used with type
  O.o Ast.T.expr O.m = O.o Ast.T.expr Mon.m
Type
  O.o Ast_js.T.expr =
    [ `Binop of To.binop * To.expr * To.expr
    | `Cst of To.constant
    | `Ecall of To.ident * To.arg list
    | `Fun of To.ident list * To.bloc
    | `Lval of To.lvalue
    | `Unop of To.unop * To.expr ]
is not compatible with type
  O.o Ast.T.expr =
    [ `Binop of To.binop * To.expr * To.expr
    | `Cst of To.constant
    | `Ecall of To.ident * To.arg list
    | `Epos of Ast.location * To.expr
    | `Fun of To.ident list * To.bloc
    | `Lval of To.lvalue
    | `Unop of To.unop * To.expr ]
The first variant type does not allow tag(s) `Epos

I am a little confused as to why this coercion is impossible... It
looks like a simple upward cast to me.
Cheers,
Till