Fraser fir this purpose. Fraser fir has been used more times as the White House Christmas tree (the official Christmas tree of the President of the United States's White House) than any other tree. It is grown in plantations in Scotland and sold by the thousands throughout the United Kingdom and Ireland. It is also cultivated from seedlings in several northern states in the USA and adjacent parts of Quebec province, especially for the Christmas tree trade. The combination of form, needle retention, dark blue-green color, pleasant scent and excellent shipping characteristics has led to Fraser fir being a most popular Christmas
Tree walking automaton Tree walking automaton A tree walking automaton (TWA) is a type of finite automaton that deals with tree structures rather than strings. The concept was originally proposed by Aho and Ullman. The following article deals with tree walking automata. For a different notion of tree automaton, closely related to regular tree languages, see branching automaton. All trees are assumed to be binary, with labels from a fixed alphabet Σ. Informally, a tree walking automaton "A" (TWA) is a finite state device which walks over the tree in a sequential manner. At each moment "A" visits a node "v" in state