Return weight of formula element.

    This implements the default weight proposed for MapMaker.

def default_weight(element):
    """Return weight of formula element.

    This implements the default weight proposed for MapMaker.
    """
    if element in (Atom.N, Atom.O, Atom.P):
        return 0.4
    elif isinstance(element, Radical):
        return 40.0
    return 1.0