If ▁v ▁= ▁ ⊥ ▁, ▁then ▁there ▁can ▁be ▁no ▁other ▁nodes , ▁and ▁F ▁represents ▁ Ø , ▁the ▁empty ▁family . ▁▁ 2 . <0x09> If ▁v ▁= ▁ ⊤ , ▁then ▁there ▁can ▁be ▁no ▁other ▁nodes , ▁and ▁F ▁represents ▁the ▁family ▁containing ▁just ▁the ▁empty ▁set ▁{ ▁ Ø ▁} . ▁We ▁call ▁this ▁a ▁unit ▁family , ▁and ▁denote ▁it ▁by ▁. ▁ 3 . <0x09> If ▁v ▁has ▁two ▁children . ▁Let ▁v 0 ▁be ▁the ▁LO ▁node , ▁and ▁v 1 ▁be ▁the ▁H I ▁node . ▁Let ▁F i ▁be ▁the ▁family ▁represented ▁by ▁the ▁Z DD ▁root ed ▁at ▁vi , ▁which ▁can ▁be ▁shown ▁by ▁proof ▁of ▁induction . ▁Then ▁F ▁represents ▁the ▁family ▁▁ ▁One ▁may ▁represent ▁the ▁LO ▁branch ▁as ▁the ▁sets ▁in ▁F ▁that ▁don ' t ▁contain ▁v : ▁ ▁And ▁the ▁H I ▁branch ▁as ▁the ▁sets ▁in ▁F ▁that ▁do ▁contain ▁v : ▁ ▁Example ▁ ▁Figure ▁ 3 : ▁The ▁family ▁. ▁We ▁may ▁call ▁this ▁, ▁an ▁elementary ▁family . ▁Element ary ▁families ▁consist ▁of ▁the ▁form ▁, ▁and ▁are ▁denoted ▁by ▁. ▁ ▁Figure ▁ 4 : ▁The ▁family ▁▁ ▁Figure ▁ 5 : ▁The ▁family ▁▁ ▁Figure ▁ 6 : ▁The ▁family ▁ ▁Fe atures ▁One ▁feature ▁of ▁Z DD s ▁is ▁that ▁the ▁form ▁does ▁not ▁depend ▁on ▁the ▁number ▁of ▁input ▁variables ▁as ▁long ▁as ▁the ▁combination ▁sets ▁are ▁the ▁same . ▁It ▁is ▁unnecessary ▁to ▁fix ▁the ▁number