code: void addExpectedPolicy(String expectedPolicy) {
          if (isImmutable) {
              throw new IllegalStateException("PolicyNode is immutable");
          }
          if (mOriginalExpectedPolicySet) {
              mExpectedPolicySet.clear();
              mOriginalExpectedPolicySet = false;
          }
          mExpectedPolicySet.add(expectedPolicy);
      }
docs: Adds an expectedPolicy to the expected policy set.
  If this is the original expected policy set initialized
  by the constructor, then the expected policy set is cleared
  before the expected policy is added.
  
  @param expectedPolicy a String representing an expected policy.