One may even want to consider proce

One may even want to consider process theories with subtraction, where for everyprocess f , there exists another process −f such that f + (−f) = 0. As usual, we canabbreviate f + (−g) as f − g. In terms of overlays this can be thought of as a layer that‘neutralises’ another layer. Note that, by distributivity, this is equivalent to assuming aspecial number ‘−1’ such that 1 − 1 = 0. If we include subtraction, the numbers of theprocess theory form a ring. Without subtraction, they form a weaker kind of structure,sometimes called a unital semiring or rig (because it is a ring without negative numbers).