Sorry for the title, it is not very evident and intuitive but I really do not how to tell it better...
Well, you know, several times, or better, many times, we use this form:
If I want to say:
"development of special weapons was the first point in Hitler's program..."
I will say this (a better form):
"special weapon development was..."
"special weapon development"
"special weapons development" (note the plural...)
what's the grammar rule in order to understand how to use this very used form?