val (**/) : num -> num -> num 
 val power_num : num -> num -> num
   Exponentiation

The num library documentation doesn't seem to support modular
exponentiation. This is seems to be an unusual oversight, and means
the library isn't useful for real-world cryptographic applications.

Is anyone working on adding mod exp's? Or are there other bigint
ocaml libraries?

Cheers,
Julian.