Reference for Arakhin 7:13

הכל מצטרפין לזימון כהנים לוים וישראלים

I might have assumed that since the commoner, even though he desired to eat with the priest [of the latter's food], he could not do so, therefore he could not be joined to him [for the zimmun] either, so we are informed that granted that the non-priest may not eat together with the priest, the priest could surely eat together with the non-priest.<span class="x" onmousemove="('comment',' It need not be mutually possible to join in the meal, hence as long as priest and non-priest are able to partake of one meal together, the zimmun is obligatory, for even the priest is permitted to eat non-consecrated food.');">⁶</span>

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