https://github.com/KoslickiLab/dnds-using-fmh/blob/cd73637bf5ae85963c78b478399f4e2bc3480a08/src/sourmash_dnds_estimation/dNdS/create_ground_truth_file.py#L109 Otherwise, you're mutating p % of the time, and not mutating p% of the time as well (and we want not mutating 1-p% of the time)