To recapitulate, the occurrence of middles in the present tense with a generic meaning is not a hard-and-fast constraint.
The reason why no hard-and-fast rules can be found is simply that the distinctions in question are not clear-cut.
In some cases this is due to the fact that the observed differences in behaviour have been shown to be tendencies and, as such, cannot be captured in hard-and-fast rules.
Customs may grow rigid so that, instead of getting increasing integration we get hard-and-fast dividing lines.
There is no hard-and-fast ceiling set at two years.
He thought that there should be no hard-and-fast limit, but that there should be variations.
There is no hard-and-fast conclusion one way or the other.
There is no hard-and-fast line like this which can be drawn.
