The filter construction is based on an all-pass which performs a reversal of
phase at the filter frequencies. Thus, the mean of the phase-distorted and
the original signal has the respective frequencies removed.
References
Pei, Soo-Chang, and Tseng, Chien-Cheng "IIR Multiple Notch Filter
Design Based on Allpass Filter"; 1996 IEEE Tencon, doi:
tools:::Rd_expr_doi("10.1109/TENCON.1996.608814")