It's a "subset of general graphs - the restriction is that a node may have only one incoming and one outgoing edge with a given edge label. So structures are or

It's a "subset of general graphs - the restriction is that a node may have only one incoming and one outgoing edge with a given edge label. So structures are organized as lists/strings of nodes, which makes it easier to visualize than general graphs" http://lambda-the-ultimate.org/node/233#comment-1706 http://lambda-the-ultimate.org/node/233#comment-1706