Generates antichains from a DAG.
An antichain is a subset of a partially ordered set such that any two elements in the subset are incomparable.
G (NetworkX DiGraph) – Graph
NetworkXNotImplemented– If G is not directed
NetworkXUnfeasible– If G contains a cycle
This function was originally developed by Peter Jipsen and Franco Saliola for the SAGE project. It’s included in NetworkX with permission from the authors. Original SAGE code at:
 Free Lattices, by R. Freese, J. Jezek and J. B. Nation, AMS, Vol 42, 1995, p. 226.