There are two kinds of dependence exhibited by computations over this tree: dependence of an operator upon its operands, and dependence due to side effects on some shared object. Computation of expression values (Section 3.2.1) illustrates operator/operand dependence. In Appel's language, the only two shared objects manipulated by programs are the memory and the output stream. Section 3.2.2 shows how the dependences arising from those objects is taken into account.