| <:ctyp< $id:id$ >> ->
       (match id with
          | <:ident< $tp1$ . $tp2$ >> ->
            (match tp_path tp2 with [n] -> n | _ -> assert false) ::  
tp_path tp1
          | _ -> invalid_arg "tp_path")

<:ident ... >> above apparently made tp1 and tp2 into idents. How do  
I convert them to ctyp as the error below requires?

This expression has type
   Camlp4.PreCast.Syntax.Ast.ident = Camlp4.PreCast.Ast.ident
but is here used with type
   Camlp4.PreCast.Syntax.Ast.ctyp = Camlp4.PreCast.Ast.ctyp

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