Syntax and implementation for converging pipelines

[gaow]

As discussed during the meeting, instead of keep branching out we want a syntax to converge the pipelines. For example,

A * B * C > D

where all results from C are consolidated at D. But it gets tricky when D depends on both C and B, and convergence of B on D is also expected at this point. Implementation-wise it is not too much difference from implementing it for C, but syntax is a bit obscure for "B converging to D" in this case.