One of my faithful readers asked for a meatier description of the thing I described in the previous post. Here goes.
Let's consider a system that can be decomposed into four binary features, A, B, C and D. Let's pretend we're building a verbal system, so this is some kind of TAM++ system we're describing. So, let's consider what the four things could be:
A: transitive vs. intransitive
B: perfective vs. imperfective
C: past vs. non-past
D: irrealis vs. realis
However, the markers do not simply mark a single feature at a time, so the markers that exist may be:
א perfective + transitive + realis + past
ב past + perfective
ג irrealis + intransitive
ה past + intransitive
ו imperfective + intransitive
The idea is there should be more of these, but working out a nice system is tedious so I skipped that bit. With the system above, if you want to express transitive irrealis, you have to construct it using ג and ד - irrealis intransitive + transitive, where transitive overrules the intransitive bit of the previous morpheme. Thus the system is fusional, in part, and agglutinating in part.