def lremove(string, prefix):
    """
    Remove a prefix from a string, if it exists.
    >>> lremove('www.foo.com', 'www.')
    'foo.com'
    >>> lremove('foo.com', 'www.')
    'foo.com'
    """
    if string.startswith(prefix):
        return string[len(prefix):]
    else:
        return string