## The Combination struct

A combination is stored by a structure containing three components, the values of n and k, and a pointer to the combination array. The elements of the combination array are all of type `size_t`, and are stored in increasing order. The `gsl_combination` structure looks like this,

    typedef struct
    {
      size_t n;
      size_t k;
      size_t *data;
    } gsl_combination;