  Bimonoidal categories (also known as rig categories) are categories with two
monoidal structures, one of which distributes over the other. We formally
define sheet diagrams, a graphical calculus for bimonoidal categories that was
informally introduced by Staton. Sheet diagrams are string diagrams drawn on a
branching surface, which is itself an extruded string diagram. Our main result
is a soundness and completeness theorem of the usual form for graphical
calculi: we show that sheet diagrams form the free bimonoidal category on a
signature.
