1. A method of modifying a sub-circuit layout, the sub-circuit used by a plurality of circuits, the method comprising: defining at least one context for the sub-circuit for each circuit that uses the sub-circuit; in the case that a plurality of contexts are defined, minimizing a number of contexts for the sub-circuit by combining contexts into at least one stage; placing each stage into a staged layout; and modifying the sub-circuit by modifying the staged layout.