code: static Matcher findProperLabelMatcher(MatcherPath path, int errorIndex) {
          try { return findProperLabelMatcher0(path, errorIndex); }
          catch(RuntimeException e) {
              if (e == UnderneathTestNot) return null; else throw e;
          }
      }
docs: Finds the Matcher in the given failedMatcherPath whose label is best for presentation in "expected" strings
  of parse error messages, given the provided lastMatchPath.
  
  @param path       the path to the failed matcher
  @param errorIndex the start index of the respective parse error
  @return the matcher whose label is best for presentation in "expected" strings