from typing import Counter


def find_most_common_prefix(txt):
    """
    Find the most common prefix (prefix is partitioned by /)
    in txt split into lines
    """
    outlines = (line.strip() for line in txt.decode("utf-8").split("\n"))
    ctr = Counter()
    for line in outlines:
        ctr[line.partition("/")[0]] += 1
    return ctr.most_common()[0][0]