Gaussian elimination, Mason's rule naturally collects these into a single quasi-inverse. General form is Where as described above, is a sum of cycle products, each of which typically falls into an ideal (for example, the strictly causal operators). Fractions of this form make a subring of the rational function field. This observation carries over to the noncommutative case, even though Mason's rule itself must then be replaced by Riegle's rule. See also Signal-flow graph Riegle's rule Notes References Control theory