10

Ranked Trees

A
ranked alphabet is a set S and r : S ! N

For a 2 S, r = r(a) Έ 0
is its *rank; *also write ar

Rank(S)=max(r(a) | a 2 S)

A
ranked tree T is a tree labeled with symbols in S s.t. each
node x labeled with a has exactly r(a) children