struct PK_TOPOL_fcstr_edge_fin_s
       {
       int  edge;
       int   fin;
       };
typedef struct PK_TOPOL_fcstr_edge_fin_s
               PK_TOPOL_fcstr_edge_fin_t;
struct PK_TOPOL_fctab_edge_fin_s
       {
       PK_TOPOL_fcstr_edge_fin_t   *data;
       int                        length;
       };
typedef struct PK_TOPOL_fctab_edge_fin_s
               PK_TOPOL_fctab_edge_fin_t;


This is a support structure for PK_TOPOL_facet_r_t. It represents
a two column table which maps fin indices into edge entity values.
(i.e. it defines which fin is derived from which PK edge entity).
Note that the mapping from fin to edge is a sparse relation,
where interior fins have no association with PK edges. The
two column table provides a compact notation for such data.