[prof1515]

And should it interest you as to the development of the RPI Family, here's a rough breakdown.

Original Three:

Armageddon
Harshlands
Forever's End (derived from Harshlands)

Derived from Harshlands:

Forever's End
Shadows of Isildur

Derived from Forever's End

Forever Ends (FEM2)
Forever's End 3
Chronicles of Ritnarium
Shadows and Mist
Dark Horizon

Derived from Shadows of Isildur or Project Argila (a clean-up of the SoI RPI Engine):

Dark Sun
Death's Omen
Hadrian's Wall
Stone MUD
Syrian Sands
Camino Real
Dead6
Mythic Shores
The Streets of Yesterday
Wild West RPG
World of Vale
Subterranea
(and the other two whose names escape me)

Armageddon and Southlands each had their own independent code development.