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Double-pushout (DPO) representation of the application of a graph transformation rule. The actual reaction (the top span 
) is the Meisenheimer rearrangement which transforms educt graph G into product graph H. All arrows in the diagram are morphisms, i.e. functions which map vertices/edges from the graph on the arrow tail to the graph at the arrow head. To be a valid transformation, the two squares of the diagram must form so-called pushouts.
