about the hybridisation and gui
in my eyes many many many possibilities must be considered there, some kind of node system might be handy for such ...
consider the following:
lets think of we have
formula a and
formula b
all the following combinations must be covered with the gui:
abababababa.... ( plain alternation, 1xa + 1xb )
bababababab.... ( plain alternation, 1xb + 1xa )
aabaabaabaab... ( alternation with amounts: 2xa + 1xb )
abbabbabbabb... ( alternation with amounts: 1xa+2xb)
dont forget, ordering is crucial....
and any combination of such, meaning to repeat a combination forever,
but such combinations should be as well doable with the hybrid editor:
aaaaaaaaaaab.... ( begin with a for n steps, continue with b)
bbbbbbbbbba.... (begin with b for n steps, continue with a )
or
aaaaabaaaaa.... ( place formula b at a certain iteration, continue with formula a )
bbbbbabbbb.... ( continue with formula b )
or the complete arbitrary placement of formulas:
aabaaaaaba.... (continue in any manner, basically allow each iteration to use a different formula)
the organisation of such might be very time consuming and slowing down a final render, but hey, the result is what counts

AND NOW COMES THE MOST CRUCIAL PART:
any hybrid has to be selectable/saveable as new base formula!!!!