Raise an exception, if there are different amount of specified occurrences in src.

def _assert_occurrence(probe, target, amount=1):
    '''
    Raise an exception, if there are different amount of specified occurrences in src.
    '''
    occ = len(probe)
    if occ > amount:
        msg = 'more than'
    elif occ < amount:
        msg = 'less than'
    elif not occ:
        msg = 'no'
    else:
        msg = None

    if msg:
        raise CommandExecutionError('Found {0} expected occurrences in "{1}" expression'.format(msg, target))

    return occ