Set item for `Presentation.rules` does nothing

[james-d-mitchell]

If we do:

p = Presentation("abc")
p.rules = ["aaaa"]
p.rules[0] = "bbb"
p.rules
["aaaa"]

I think either we should make p.rules assignable (not sure if this is feasible) or it should throw an exception, silently failing like this caused me some confusion.