Non-deterministic Feedback Combinator

My thesis has shown the usefulness of a non-deterministic feedback combinator.
Also later work on Cholesky Decomposition confirmed that some programs
can only be implemented efficiently with a non-deterministic feedback.
The snetc compiler already differentiates a single-backslash from a double-backslash
as does the recently revised S-Net Language Report 2.1.
For the double-backslash it will call the old feedback implementation and for the single
 this new one.
The provided implementation is both much smaller in size and I believe also simpler.
Tests show that it can be very much faster. Elaborate implementation details are
part of the source file feednond.c.