def mapcat(fn, colls):
return concat(map(fn, colls))<EOL>
Concatenate the result of a map

    :param fn: a function
    :param colls: a list of collections
    :returns: a list

    >>> mapcat(reversed,  [[3, 2, 1, 0], [6, 5, 4], [9, 8, 7]])
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]