by convention, the tree is drawn with the root at the top and the leaves toward the bottom (an "uprooted" tree). problem 9.4 shows that trees can also be used to describe the structure of arithmetic expressions.
A tree may be described by assigning ist nodes to numbered levels: the root is put in level 1; nodes joined by a branch to the root are put in level 2; and so on. figure 9-2 shows the level structure of the tree of Fig. 9-1 (b)