Christmas tree Macy's Great Tree in Atlanta (since 1948), the Rockefeller Center Christmas Tree in New York City, and the large Christmas tree at Victoria Square in Adelaide. The use of fire retardant allows many indoor public areas to place real trees and be compliant with code. Licensed applicants of fire retardant solution spray the tree, tag the tree, and provide a certificate for inspection. Real trees are popular with high end visual merchandising displays around the world. Leading global retailers such as Apple often place real trees in their window displays. In 2009, Apple placed two Fraser fir trees in every
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