Now my job is dying here with this PR in place (so new bug)

```clojure
java.lang.IllegalArgumentException: cascalog.logic.def.ParallelAggregator@151a252c is an invalid predicate.
        at jackknife.core$throw_illegal.doInvoke(core.clj:155)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at cascalog.logic.predicate$eval6235$fn__6236.invoke(predicate.clj:162)
        at clojure.lang.MultiFn.invoke(MultiFn.java:236)
        at cascalog.logic.predicate$build_predicate.invoke(predicate.clj:230)
        at clojure.lang.AFn.applyToHelper(AFn.java:163)
        at clojure.lang.AFn.applyTo(AFn.java:151)
        at clojure.core$apply.invoke(core.clj:619)
        at clojure.core$partial$fn__445.doInvoke(core.clj:2396)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$map$fn__466.invoke(core.clj:2485)
        at clojure.lang.LazySeq.sval(LazySeq.java:42)
        at clojure.lang.LazySeq.seq(LazySeq.java:60)
        at clojure.lang.ChunkedCons.chunkedNext(ChunkedCons.java:59)
        at clojure.core$chunk_next.invoke(core.clj:660)
        at clojure.core.protocols$eval2831$fn__2832.invoke(protocols.clj:101)
        at clojure.core.protocols$eval2771$fn__2772$G__2762__2781.invoke(protocols.clj:19)
        at clojure.core.protocols$seq_reduce.invoke(protocols.clj:31)
        at clojure.core.protocols$eval2802$fn__2803.invoke(protocols.clj:54)
        at clojure.core.protocols$eval2735$fn__2736$G__2726__2749.invoke(protocols.clj:13)
        at clojure.core$reduce.invoke(core.clj:6177)
        at clojure.core$group_by.invoke(core.clj:6492)
        at cascalog.logic.parse$build_rule.invoke(parse.clj:637)
        at cascalog.logic.parse$parse_subquery.invoke(parse.clj:687)
        at ybot.analytics.donbot.jobs.language_model$_main$fn__11318$fn__11333.invoke(language_model.clj:90)
        at cascalog.checkpoint$mk_runner$fn__10016.invoke(checkpoint.clj:60)
        at clojure.lang.AFn.run(AFn.java:24)
        at java.lang.Thread.run(Thread.java:662)
```