Felix: that still has the problem of misleading players in a sequence like >JUMP (“You are unable to do that.”) when in fact >JUMP OVER CHASM is what they need to do next. The difficulty is the conflation of you the player (“You are unable to use that command.”) with you the player character (“Bob is unable to jump.”) Maybe it would be more accurate to invite the parser in and say something like “I am unable to direct your character to do that.” But that’s kind of a mouthful.
Matt: To me, “nothing obvious happens” always implied that something terribly interesting was going on somewhere else, if only I could figure out what this seemingly innocuous thing I pushed was connected to. Maybe that’s just me… good points about the interpreter, I’ll rephrase this.