* wrapper around j.u.HashSet, which has built-in flooding protection. MAX_RUN_MULTIPLIER was
     * determined experimentally to match our desired probability of false positives.
     */
    // NB: yes, this is surprisingly high, but that's what the experiments said was necessary
    // Raising this number slows the worst-case contains behavior, speeds up hashFloodingDetected,
    // and reduces the false-positive probability.