No check available to verify if IP used for join operations is capable of doing it when bottom branch is a direct connection

[mariodruiz]

Assuming this pipeline

[cpipe.f1, cpipe.fork, [[cpipe.f3],[1]], cpipe.f5, cpipe.f2]

If cpipe.f5 does not have two mi entries in the c_dict the above pipeline is not feasible.
The compose method, will return a generic error IndexError: list index out of range, which is not indicative of what it is really happening.

For these other pipeline:

[cpipe.f1, cpipe.fork, [[cpipe.f3], [cpipe.f4]], cpipe.f5, cpipe.f2]
[cpipe.f1, cpipe.fork, [[1], [cpipe.f4]], cpipe.f5, cpipe.f2]

The correct exception is thrown: SystemError: Node f2 has 1 input(s) and cannot meet pipeline requirement of 2 input(s)