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 a
r
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