One can insert the particle quippe in a relative clause to give it a causal or otherwise explaining tone.1 Does quippe require using conjunctive in the relative clause? If not, are there some rules for choosing between indicative and conjunctive that are special to quippe? Causal relative clauses usually have conjunctive, but I wonder if quippe is enough for expressing causality and the conjunctive can be omitted.
Other words can also be used to emphasize a causal nature of a relative clause: ut, utpote and praesertim. I suspect these behave like quippe as far as using conjunctive is concerned.
After reading my grammar once again while preparing this question, I am inclined to believe that conjunctive is obligatory with quippe in a relative clause, but I want to have my suspicions clearly confirmed or refuted.
This question originated from our chat room. When chatting about using three languages at the same time, I remarked: Sed crediderim id facilius esse Cerbero, quippe qui tria capita habet. Cerberus questioned my choice of indicative, and since I couldn't find a definitive answer elsewhere, I had to ask this question.
1 I have only ever seen quippe in this use. If you know other uses, please answer this question. I have explained the causal use in my answer2 to that question.
2 I may have to update that answer depending on the answer I get here.