def postfix_concatenate(op, left, right):
    """ Concatenate arguments placing op on the middle.

    :example:

    >>> postfix_concatenate('+', 1, 2)
    [1, 2, '+']
    >>> p = postfix_concatenate('+', 1, 2)
    >>> postfix_concatenate('*', p, 3)
    [1, 2, '+', 3, '*']
    >>> postfix_concatenate('*', 3, p)
    [3, 1, 2, '+', '*']
    """
    if isinstance(left, list):
        left_tokens = left
    else:
        left_tokens = [left]
    if isinstance(right, list):
        right_tokens = right
    else:
        right_tokens = [right]
    return left_tokens + right_tokens + [op]
