stronger/weaker stays

[onsetsu]

Sometimes certain variables should stay stronger on its value than others. Adding a notion of strength for stay constraints would allow the user to specify this.

[timfel]

yes, I'm just not sure how to write this. Babelsberg/R has always { x.stay :required }, but that is really confusing and strange. Maybe just obj.stay('x', 'required') works, but it also looks strange.