def is_edge(s):
    """Return True if s is a description of the base pair edges.
    
    s -- string

    X is added to the list of accepted symbols. X might be used
    for modified bases.
    """
    chars = dict.fromkeys("WHsS+-?.X")
    try:
        first, second = s.split('/')
    except ValueError:
        return False
    if first not in chars or second not in chars:
        return False
    return True