opam switch add-invariant

[kit-ty-kate]

In this discuss post i realized that the current way to add a package or a formula to the current invariant isn't really straightforward:

opam switch set-invariant --formula "$(opam switch invariant | sed 's/]/ \"<pkg>\"]/')"

To remedy this i propose adding the following sub-command:

opam switch add-invariant

which works the exact same way as opam switch set-invariant but instead of setting the invariant it's adding to the invariant (concatenate the two formula)

One can also think of the reverse: opam switch remove-invariant which would scan the whole formula for the given formula or package and remove the matching subformula or package