[r-t] Changes generating groups

[ALEX BYRNE]

Ander wrote: The other examples I know of are:
Minor:
<-,1,34> : Kidderminster
<-,3,4> : Striking

I think there's Eureka Minor as well (although I don't know the changes).

What are the properties of a set of changes that generate a group? Is it
obvious that <-,1,34> works?

In the same vein, what are the fundamental (group?) properties of methods
that enable an extent to be produced? It's not at all intuitive to me that
Cambridge Minor, say, can produce all 720 rows in such an elegant way.

Alex.