(Responses) Provide a way of tagging which responses are parser errors
In adapting the Standard Rules and extensions for Inform 6L02, we've followed the convention that parser error messages to the player (rather than to/about the protagonist) should remain second person and present tense, or else be phrased as neutrally as possible, rather than adapting to the viewpoint used for other messages.
However, some authors are concerned about a potentially confusing shift between second person and other persons of narration; others would like to be able to make systematic changes to how all parser errors are shown, for instance by setting them off in brackets or italics, or instructing Vorple to remove them from view after a turn of play.
The current "before printing a parser error" activity allows authors to intervene in errors printed by the Standard Rules, but not those included in extensions. If, however, it were possible to mark responses in extensions as also being parser errors, we could use the same hooks to apply parser error styling choices across all included extensions as well as the Standard Rules.