1. A computer executed method, comprising: accepting an original circuit, wherein said original circuit has an original logic, and wherein said original logic comprises primary output elements; accepting a modified circuit, wherein said modified circuit represents a modified logic for said original circuit; synthesizing a difference circuit, wherein said difference circuit represents changes to said original logic, wherein said changes implement said modified logic for said original circuit; and wherein said synthesizing comprises: locating an output-side boundary in said original logic, wherein inbetween said output-side boundary and said primary output elements said original logic is free of said changes.