Graph expression The manual for the LaTeX package TikZ v. 3.0.1a states (section 19.7 "Graph Operators, Color Classes, and Group Expressions", p. 280): > TikZ's graph command employs a powerful mechanism for adding edges between nodes and sets of nodes. To a graph theorist, this mechanism may be known as a _graph expression_ : A graph is specified by starting with small graphs and then applying _operators_ to them that form larger graphs and that connect and recolor colored subsets of the graph's node in different ways. I searched Google for "graph expression" and couldn't find anything relevant. Where would a graph theorist know the term _graph expression_ from? I'd appreciate references to where this term is defined, explained and used.

This book - **Recent Trends in Algebraic Development Techniques: 19th International Workshop, WADT 2008** \- defines _graph expression_ and discusses operators in that context. Link