A PROP is a symmetric monoidal category, whose set of objects is the set of
natural numbers and on objects the monoidal structure is given by the addition.
An algebra over a PROP is a symmetric strict monoidal functor to the tensor
category of vector spaces. We give an explicite construction of the PROP whose
category of algebras is equivalent to the category of bialgebras (= associative
and coassociative bialgebras)
