In the definition of top-down and bottom up, both are defined as but the symbol is never defined.
I wonder why. I suppose must be the alphabet of symbols over which the tree automaton is defined. Moreover, the definition given in this page doesn't seem to be consistent with the one given in the TATA link that seems to be the reference. If nobody objects, I will edit this page in a week to update the definitions to those given in Chapter 1 of TATA.(jabial)
I updated the page as promised. --Jabial 13:44, 20 April 2006 (UTC)