struct PK_TOPOL_fctab_fin_fin_s
        {
        int     *fin;
        int   length;
        };
typedef struct PK_TOPOL_fctab_fin_fin_s
               PK_TOPOL_fctab_fin_fin_t;


This is a support structure for PK_TOPOL_facet_r_t. It represents a one column
table which maps fin indices onto co_fins. Fins without co_fins (laminar edges
on sheet bodies or edges that lie on loops that are being ignored on one face)
have their co_fin value set to -1. Fins with a single co-fin
share the same start & end vertices as their co-fin but in reversed order.
This mapping is commutative.