Dario Teixeira wrote:
>     and convert_super_node node = match node with
>         | #nonlink_node_t   -> (convert_nonlink_node node :> super_node_t)
>         | #link_node_t      -> (convert_link_node node :> super_node_t)

You need to rebind the matched value within the pattern in order to 
refine the type on the rhs:

and convert_super_node node = match node with
| #nonlink_node_t as node -> (convert_nonlink_node node :> super_node_t)
| #link_node_t    as node -> (convert_link_node node :> super_node_t)

Jeremy.