https://blog.logos.com/wp-content/uploads/2005/11/syntax_andersen.html Consider the simple graph to the right. A graph, you will recall, is a diagram made up of labels and lines. This particular graph has some further special characteristics: (1) This is a directed graph, because the lines are arrows that indicate which labels are “on top,” so to speak; if this were a corporate organization chart, the arrows would always point from manager to employee. (2) This graph is acyclic, which is a fancy word meaning “no cycles,” which is a fancy way of saying that if you follow the arrows in the direction they are pointing, you will never visit the same label twice. Put another way, if no matter where you start, you will eventually reach the end. (3) This particular graph is a tree, because it has exactly one topmost label (the CEO in our org chart), and each label has one and only one arrow that points to it. That is, each employee has only one boss — wouldn’t that be nice?
I think that I shall never seea graph as lovely as a tree.