▁The ▁episode ▁is ▁named ▁after ▁the ▁Apol lo ▁ 9 ▁L un ar ▁Module . ▁ ▁Notes ▁ ▁References ▁ ▁Cit ations ▁ ▁Bibli ography ▁ ▁External ▁links ▁ ▁Category : Space fl ight ▁concepts ▁Category : A pol lo ▁program ▁Category : Space ▁rende z vous <0x0A> </s> ▁In ▁mathematics , ▁a ▁mon oid al ▁category ▁( or ▁tensor ▁category ) ▁is ▁a ▁category ▁ ▁equ ipped ▁with ▁a ▁b if un ctor ▁ ▁that ▁is ▁associ ative ▁up ▁to ▁a ▁natural ▁isomorphism , ▁and ▁an ▁object ▁I ▁that ▁is ▁both ▁a ▁left ▁and ▁right ▁identity ▁for ▁ ⊗ , ▁again ▁up ▁to ▁a ▁natural ▁isomorphism . ▁The ▁associated ▁natural ▁is omorph isms ▁are ▁subject ▁to ▁certain ▁coh er ence ▁conditions , ▁which ▁ensure ▁that ▁all ▁the ▁relevant ▁diag rams ▁comm ute . ▁ ▁The ▁ordinary ▁tensor ▁product ▁makes ▁vector ▁spaces , ▁ab elian ▁groups , ▁R - modules , ▁or ▁R - al gebras ▁into ▁mon oid al ▁categories . ▁Mon oid al ▁categories ▁can ▁be ▁seen ▁as ▁a ▁general ization ▁of ▁these ▁and ▁other ▁examples . ▁Every ▁( small ) ▁mon oid al ▁category ▁may ▁also ▁be ▁viewed ▁as ▁a ▁" c ategor ification " ▁of ▁an ▁underlying ▁mon oid , ▁namely ▁the ▁mon oid ▁whose ▁elements ▁are ▁the ▁isomorphism ▁classes ▁of ▁the ▁category ' s ▁objects ▁and ▁whose ▁binary ▁operation ▁is ▁given ▁by ▁the ▁category ' s ▁tensor ▁product . ▁ ▁A ▁rather ▁different ▁application , ▁of ▁which ▁mon oid al ▁categories ▁can ▁be ▁considered ▁an ▁abstra ction ,