  The basic principle of graph rewriting is the stepwise replacement of
subgraphs inside a host graph. A challenge in such replacement steps is the
treatment of the patch graph, consisting of those edges of the host graph that
touch the subgraph, but are not part of it.
  We introduce the patch graph rewriting framework, a visual graph rewriting
language with precise formal semantics. The language has rich expressive power
in two ways. Firstly, rules can flexibly constrain the permitted shapes of
patches touching matching subgraphs. Secondly, rules can freely transform
patches. While the framework is designed to be easy to understand, it subsumes
many approaches to graph rewriting.
