OK, Without ASL below are the base permutations. From that let's eliminate some obvious "I doubt anyone would do this"
Based on Russells comment that most bridges involve DMR
DMR <-> D-Stsr
DMR <-> YSF
DMR <-> NXDN
DMR <-> P25
D-Star <-> YSF
D-Star <-> NXDN
D-Star <-> P25 (I doubt anyone would do this)
YSF <-> NXDN
YSF <-> P25
NXDN <-> P25
BTW, this kinda ignores YSFn vs YSFw
Anyone else care to eliminate a bridge? I'm leaning to create the first four Subgroups and see what happens.