1. In a computer implemented synthesis system, a subtractive method of generating a netlist containing scan replaced sequential cells and satisfying determined optimization constraints, said method comprising the computer implemented steps of: a) accessing an input netlist containing scan replaced sequential cells; b) determining a set of critical paths within said input netlist; c) selecting a selected critical path of said set of critical paths and identifying a first sequential cell and a second sequential cell located on either end of said selected critical path; d) provided said first sequential cell and said second sequential cell are both scan replaced, determining which sequential cell of said first and second sequential cells contributes least to testability; e) within said input netlist, unscanning said sequential cell of said selected critical path that contributes least to testability; and f) repeating steps b)-e) while there are critical paths in said set of critical paths that have scan replaced cells and there are no worse critical paths that have no scan replaced cells.