a modern C++ library for the manipulation of Boolean functions

Step 1 If the node is 1 or 0 return this node.
Step 2 If the set is empty or the variable which labels this node is greater than the
maximum variable in the set return 1 (the reason for this is that all descendants
of this node are necessarily labeled with a greater variable so they must be all
projected away).
Step 3 If the variable by which this node is labeled is present in X call the algorithm
recursively on the true and false successors: the result will be the ROBDD whose
root is labeled by this variable and has the results of the two recursive calls as its
true and false successor respectively.
Step 4 Otherwise this node must be projected away. We call the algorithm recursively
on the true and false successors: the result will be the disjunction of the results of
the two recursive calls, according to Equation 1.1.
The algorithm project_threshold, which projects away all variables in a ROBDD
that are greater than a given variable, is analogous. The pseudo-code for both algorithms
is given in Algorithm 8 and Algorithm 9.
Require: a ROBDD node n and a set of variables V
function project_set(n, V )
if is_leaf(n) then
return n
else if max(V ) ≺ nvar or V = ∅ then
return 1
else if nvar ∈ V then
return make_node nvar, project_set(ntrue, V ), project_set(nfalse, V )
else
return or_ite project_set(ntrue, V ), project_set(nfalse, V )
Algorithm 8: The project_set function
Require: a ROBDD node n and a threshold variable t
function project_threshold(n, t)
if is_leaf(n) then
return n
else if t ≺ nvar then
return 1
else
return make_node nvar, project_threshold(ntrue, t), project_threshold(nfalse, t)
Algorithm 9: The project_threshold function
3.2 ROBDD algorithms 21