A
monoidal category structure on the category of quivers.
This consists of a product bifunctor, a unit object and structure
morphisms, an invertible associator,
qassoc . qdisassoc = id
qdisassoc . qassoc = id
and invertible left and right unitors,
qintro1 . qelim1 = id
qelim1 . qintro1 = id
qintro2 . qelim2 = id
qelim2 . qintro2 = id
that satisfy the pentagon equation,
qbimap id qassoc . qassoc . qbimap qassoc id = qassoc . qassoc
and the triangle equation,
qbimap id qelim1 . qassoc = qbimap qelim2 id