Topological Sort
A topological sort is a permutation 
of the vertices
of a graph such that an edge 
implies that
appears before 
in 
(Skiena 1990, p. 208).
Only acyclic digraphs can be topologically sorted.
The topological sort of a graph can be computed using TopologicalSort[g]
in the Wolfram Language package Combinatorica`
.
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Ammann A4 tiling
