@Override
          protected ImmutableSet<FactoryMethodDescriptor> defaultAction(Element e, Void p) {
            throw new AssertionError("@AutoFactory applied to an impossible element");
          }
A service that traverses an element and returns the set of factory methods defined therein.

@author Gregory Kick
factory/src/main/java/com/google/auto/factory/processor/FactoryDescriptorGenerator.java