  Morphisms in a monoidal category are usually interpreted as processes, and
graphically depicted as square boxes. In practice, we are faced with the
problem of interpreting what non-square boxes ought to represent in terms of
the monoidal category and, more importantly, how should they be composed.
Examples of this situation include lenses or learners. We propose a description
of these non-square boxes, which we call open diagrams, using the monoidal
bicategory of profunctors. A graphical coend calculus can then be used to
reason about open diagrams and their compositions.
