can “must” be added in English to the rules DID_BASEFORM?
The following, for example, is wrong and is not being caught by the grammar check:
 They must comes home
 Mozilla must has a grammar check
I’m not sure why there are 4 subversions of this rule. But I suppose the rule can apply to all of them, even though mustn’t  is quite rare and using must in the context of  is quite confusing, for example.