Omit unnecessary arcs from the executable graph

[emil14]

A note from "dataflow and reactive programming" book:

First, let’s look at the easy situation using compound nodes in static
dataflow systems. Since static dataflow programs are not allowed
to change at run-time, we can view compound nodes as simply a
design-time convenience to make building and understanding the
program easy for the developer. At compile-time, compound nodes
are recursively expanded to primitive nodes and that is what the
dataflow engine executes.

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Related to #212