Other tracer techniques exist, such as negative step (i.e., elution), fre-
quency-response methods, and methods that use inputs other than steps or
pulses. These methods are usually much more difficult to carry out than the
ones presented and are not encountered as often. For this reason they will not
be treated here, and the literature should be consulted for their virtues, defects,
and the details of implementing them and analyzing the results. A good source
for this information is Wen and Fan.4