It was based on fvwm code, but it evolved independently.
What was good is that it enabled fvwm modules to be ported very easily.
Good memories from a long time ago (started doing that in 1997 ... wow! cf http://www.afterstep.org/visualdoc.php?show=copyright&sub=ol...)
I tried it but found WindowMaker more internally consistent. In fact, I'm still using it, some 16 years later.