def remove_leading(needle, haystack):
    """Remove leading needle string (if exists).

    >>> remove_leading('Test', 'TestThisAndThat')
    'ThisAndThat'
    >>> remove_leading('Test', 'ArbitraryName')
    'ArbitraryName'
    """
    if haystack[:len(needle)] == needle:
        return haystack[len(needle):]
    return haystack
