[Ur] introducing a name in functor body

[Gergely Buday]

> The problem is the same as if you had merely written [val foo : int] instead
> of [val foo = 3]: you are writing an _interface_ declaration (i.e.,
> "signature item") in a position that expects an _implementation_ declaration
> (just called "declaration" in the manual).  There is no declaration for
> generating a fresh name automatically; you would probably take that name
> into the functor as an input.

But this is a field name that I want to use independently of an input
structure. I put it outside the functor but in vain. How can I declare
this field name to be used later?

- Gergely