code: static URI checkType(final URI type) {
          Preconditions.checkNotNull(type, "No type specified");
          if (!type.equals(KS.RESOURCE) && !type.equals(KS.MENTION) && !type.equals(KS.ENTITY)
                  && !type.equals(KS.AXIOM)) {
              throw new IllegalArgumentException("Invalid type: " + type);
          }
          return type;
      }
docs: UTILITY METHODS