code: public Tristate predicateCacheEntry(final CacheEntry key) {
          final Tristate tristate = this.predicateCache.get(key);
          if (tristate == null) {
              return Tristate.UNDEF;
          }
          return tristate;
      }
docs: Returns an entry of the predicate cache of this formula.
  @param key the cache key
  @return the cache value (which is {@code UNDEF} if nothing is present)