among others, `.value_forbidden:` and `.value_required:` key properties
from `l3keys` package are useful: they let us specify that a given key,
when used, either cannot or must receive a value.

What could be useful as well is a key property, say `.required:`, that would
specify that the corresponding key /has/ to be used. This could be helpful for
instance in the case of a document command `\MyModuleSetup` for setting up
a module `mymodule`:

    \DeclareDocumentCommand \MyModuleSetup { m }
      { \keys_set:nn { mymodule } { #1 } }

where some module property /has/ to be specified.

I know this can be achieved with something like test of existence of
some tokenlist and message emitted in case of nonexistence but a high
level key property for this could be nice.

May I ask what is your opinion about that?

Thanks in anticipation.