'Count occurrences of value v in the entire history. Note that the subclass must implement the __reversed__
method, otherwise an exception will be thrown.
:param object v: The value to look for
:return: The number of occurrences
:rtype: int'

def count(self, v):

    ctr = 0
    for item in reversed(self):
        if (item == v):
            ctr += 1
    return ctr
