I admire the deceptively small scale of this Entry, an instantiation of the "tiny, hyperelegant zoo" I posited in my original challenge. The parser is limited to great effect. The pacing is great. The extent of the work, so to speak, is really impressive.
The basic mechanic of this game may seem better suited to some kind of hypertext presentation, in which the few actions available could be static buttons next to a text box with the current turn’s description. But in hypertext IF, the subtly changing text that "Indistinguishable" relies upon is de rigueur. You expect it and recognize it immediately.
In parser IF, a room description tends to be utilitarian and stable. You read it once, and then you pay it minimal attention until you're actively looking for clues. "Hiding" the content of this game in room descriptions and seemingly innocuous cycling text allows it to sneak up on you in a way that would be much more obvious and less fun in hypertext.
But the fact that there is so little for the player to do means that this ending-less zoo can't really be said to act as a "place" in the way other Entries in this Event do. This makes "Indistinguishable" a valuable point of reference in considering what contributes to the Sense of Place in IF that I am forever seeking to perfect. But it also makes it a less appealing game to my personal tastes. Therefore I award "Indistinguishable" 17 points out of 20.