1. A frequency-domain adaptive filter comprising: a first input signal line to receive an input signal; a second input signal line to receive a send signal; a plurality of memory stores, each said memory store having a frequency-domain coefficient vector; a plurality of weights, each said weight corresponding to one of said memory stores; transformation means coupled to said first input signal line for producing a plurality of frequency-domain input blocks from said input signal; response means for producing a time-domain response signal from said coefficient vectors and said frequency-domain input blocks, including first means coupled to said plurality of memory stores for multiplying the frequency-domain coefficient vector in each said memory store with the weight corresponding to said memory store to produce weighted coefficient vectors, second means for multiplying said weighted coefficient vectors with said frequency-domain input blocks to obtain product terms, means for summing together said product terms, and means for transforming said summed product terms to the time domain, thereby producing said time-domain response signal; combining means for producing a filtered send signal from said time-domain response signal and said send signal; an outgoing signal line coupled to said combining means to transmit said filtered send signal; and means for adapting said plurality of coefficient vectors, including: error means for producing a frequency-domain error vector from said filtered send signal; correlation means for producing correlation terms from said frequency-domain error vector and said frequency-domain input blocks; summing means coupled to said plurality of memory stores for adding said correlation terms to said coefficient vectors in said memory stores; and constraint means coupled to said plurality of memory stores for selecting at least one of said memory stores and for constraining the coefficient vector stored in said selected one of said memory store.