Acelot contributors ‘extended’ the notion of formal grammar
from strings into graphs that are defined to have a basic operational unit
called a “graph motif.” A graph motif
can be either a simple graph or composed of other graph motifs by concatenation,
disjunction, and repetition. A graph grammar is thus a finite set of graph
motifs. A language of a graph grammar is the set of all graphs derivable from
graph motifs of that grammar.