Error: You are trying to do a generic derivation of a generic
  class type but the number of actual generic parameters is not
  the same.
What to do: Make sure to have the proper count.
  For example HASH_TABLE [G, H] needs two type parameters:
   - HASH_TABLE [INTEGER] is not ok. Add another type parameter.
   - HASH_TABLE [INTEGER, STRING] fits: 2 type parameters
   are specified as in the base class.