▁() ▁Int sec ( P , ▁Q ) <0x09> <0x09> returns ▁() ▁D iff ( P , ▁Q ) <0x09> <0x09> returns ▁() ▁Count ( P ) <0x09> <0x09> returns ▁. ▁( number ▁of ▁elements ) ▁ ▁In ▁Z DD s , ▁there ▁is ▁no ▁NOT ▁operation , ▁which ▁is ▁an ▁essential ▁operation ▁in ▁original ▁B DD s . ▁The ▁reason ▁is ▁that ▁the ▁complement ▁set ▁ ▁cannot ▁be ▁computed ▁without ▁defining ▁the ▁universal ▁set ▁. ▁In ▁Z DD s , ▁ ▁can ▁be ▁computed ▁as ▁D iff ( U , ▁P ). ▁ ▁Alg orithms ▁Suppose ▁, ▁we ▁can ▁recurs ively ▁compute ▁the ▁number ▁of ▁sets ▁in ▁a ▁Z DD , ▁en abling ▁us ▁to ▁get ▁the ▁ 3 4 th ▁set ▁out ▁a ▁ 5 4 - member ▁family . ▁Random ▁access ▁is ▁fast , ▁and ▁any ▁operation ▁possible ▁for ▁an ▁array ▁of ▁sets ▁can ▁be ▁done ▁with ▁efficiency ▁on ▁a ▁Z DD . ▁▁ ▁According ▁to ▁Min ato , ▁the ▁above ▁operations ▁for ▁Z DD s ▁can ▁be ▁executed ▁recurs ively ▁like ▁original ▁B DD s . ▁To ▁describe ▁the ▁algorithms ▁simply , ▁we ▁define ▁the ▁procedure ▁Get node ( top , ▁P 0 , ▁P 1 ) ▁that ▁returns ▁a ▁node ▁for ▁a ▁variable ▁top ▁and ▁two ▁sub graph s ▁P 0 ▁and ▁P 1 . ▁We ▁may ▁use ▁a ▁hash ▁table , ▁called ▁un i q - table , ▁to ▁keep ▁each ▁node ▁unique . ▁Node ▁elim ination ▁and ▁sharing ▁are ▁managed ▁only ▁by ▁Get node (). ▁▁▁▁