node of type with finite number of associated disjoint tree structures of base type called subtrees from the similarity of the recursive definitions of sequences and tree structures it is evident that the sequence (listis tree structure in which each node has at most one subtree the list is therefore also called degenerate tree ab( ( ( ( ), ( , , )), ( ( ), ( , ), ( ))) cn di fig representation of tree structure by (anested sets(bnested parentheses(cindented textand (dgraph there are several ways to represent tree structure for examplea tree structure with its base type ranging over the letters is shown in various ways in fig these representations all show the same structure and are therefore equivalent it is the graph structure that explicitly illustrates the branching relationships whichfor obvious reasonsled to the generally used name tree strangely enoughit is customary to depict trees upside downor -if one prefers to express this fact differently -to show the roots of trees the latter formulationhoweveris misleadingsince the top node (ais commonly called the root an ordered tree is tree in which the branches of each node are ordered hence the two ordered trees in fig are distinctdifferent objects node that is directly below node is called (direct