bitmasks
  N_int         integer
  bit_kind      integer
  N_mask_gen    integer
  generators    integer (bitmasks_N_int*bitmasks_bit_kind/4,2,2,bitmasks_N_mask_gen)
  N_mask_ref    integer
  reference     integer (bitmasks_N_int*bitmasks_bit_kind/4,2,2,bitmasks_N_mask_ref)