Non-determinant foreign predicate

[mcculloch]

I am writing an interface to mongoDB from GNU Prolog using mongo's C driver. 
I want a non-determinant predicate to access query data via a C routine that
takes care of its own pointer called a cursor.  The cursor structure is
defined outside of the foreign function, so I don't need to call
Pl_Get_Choice_Buffer or Pl_Get_Choice_Counter.  I will call
Pl_No_More_Choice, but only when the cursor has reached the end of its data.

Does anyone know how prolog knows that a foreign predicate is
non-determinate.  Is it the "choice_size" in the foreign command, or is it
something else?  I need my foreign predicate to be non-determinate even if
it doesn't need to use a buffer.

Thanks,
John

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