A computer system capable of showing documents for users to read in a power saving mode is disclosed. Each clip has a main body having a structural member for securing to the rail, and having a pair of flanges symmetrical about a central axis. Catalyst temperature is filtered using a variable catalyst time constant corresponding to current space velocity of the exhaust gas to account for changes in the catalyst temperature attributed to NOx transient emissions. The combining network combines the set of inputs and the signal is delivered to the load resistor as a fourth order harmonic.