1. For a candidate sub-network that performs multiple output functions that are produced outside the candidate sub-network in an integrated circuit (IC) design, a method for retrieving a replacement sub-network for the multi-output function candidate sub-network, the method comprising: a) generating a parameter by: (i) identifying one of the output functions of the candidate sub-network as a first output function; (ii) using the first output function as a reference from which to derive an index for each output function of the candidate sub-network; (iii) deriving the parameter from the indices for the output functions of the candidate sub-network; b) using the parameter to retrieve a replacement sub-network from a storage structure that stores replacement sub-networks; and c) replacing the candidate sub-network in the design with the replacement sub-network.