def add_scheme_if_missing(url):
    """
    >>> add_scheme_if_missing("example.com/foo")
    'http://example.com/foo'
    >>> add_scheme_if_missing("https://example.com/foo")
    'https://example.com/foo'
    >>> add_scheme_if_missing("//example.com/foo")
    'http://example.com/foo'
    """
    if url.startswith('//'):
        url = 'http:' + url
    if '://' not in url:
        url = 'http://' + url
    return url
