Making sale_amendment (and purchase_amendment) editable from sale (or purchase)

We recently tried the newly released sale_amendment and purchase_amendment. We think that it would be useful to add amendments from the sale itself but that is only possible when there’s already an amendment in the sale because otherwise it is invisible. See: http://hg.tryton.org/modules/sale_amendment/file/632e5e9794c2/sale.py#l17

I think the invisible state should simply be declared as: Eval(‘state’) != ‘processing’

Opinions?

I do not think it makes a good user experience. Record inside a One2Many is difficult to use their workflow.
Indeed I think we should make the One2Many always readonly.
But probably having a relate on the sale would be good.