[PvK]

A few ideas:

* Knowledge and technology are two different things. Knowing the principles of doing things is different from having the machinery, infrastructure, and knowhow (skilled technicians) to actually build it.

* The MOO2 forced decisions seem to me way overdone and arbitrary, essentially a severe abstraction to force some diversity, without much reasonable basis.

* Jack Simth's points about the success of one theory blocking researching into other theories is excellent, though it's not a complete block.

* One way to achieve this in modded SE4 is to have major theoretical prerequisites which make it possible but impractical (or at least, a major trade-off) to try to research alternate branches. It would also help if the research points, costs, and tree depths were sufficient to avoid players "getting to the end and having nothing better to do with their research points".

* I think there are some other kinds of situations which could be abstractly modelled by unavailable branches, but again it would be a severe abstraction. For instance, conflicts between technological research versus religious or magical research. Realistic detail would include organizations within an empire which are opposed to research and development of rival fields. E.g. Corporate Capitalist empire with gasoline car manufacturing and oil/gas industry being opposed to developing more efficient and reliable cars, or alternative energy cars, even though the technology could in theory be developed quite quickly without that internal political obstacle.

* It would be convenient to be able to mark tech trees and areas as "obsolete", but I wouldn't want it to be a forced thing in most cases. On the other hand, it is a good argument that some technologies coming into prevalence can cause older technologies to fall out of use, with side-effects. Again, an abstract mechanic could represent this by having say, plate armor and cavalry become unavailable, or at least much harder to produce, after gunpowder weapons reach an advanced state. It's not really an accurate cause and effect, however. The real cause is a shift in infrastructure and trained workforce (technology) and socio-economic effects, neither of which are really included in SE4.

PvK