The case of an eruv for a kohen who can eat the terumah (which is pure), and he himself is pure, but the location for the eruv is on a grave (which is impure). Ah, but the terumah has to be prepared or readied to be eligible to be rendered impure. The Gemara gets into a number of details pertaining to the care the kohen must take - an example of the far-reaching permutations for the case, the kind that we have seen fewer of in Eruvin. Also, the Gemara makes the point that we do not do mitzvot for the sake of our personal benefit, but for the sake of doing a mitzvah. Note that this gets complicated implications for prohibited items... Also, an entirely new area of discussion -- using a shaliach, or emissary, to do the mitzvah on your behalf. But what about someone who doesn't accept that the mitzvah is a mitzvah? Or that any mitzvot are binding? What if your shaliach were a monkey or an elephant - will that work? With differences here between the erev chatzerot vs. the eruv techumin.