code: private boolean nextSegment() {
          assert(state == State.ITER_CHECK_FWD);
          // The input text [start..(iter index)[ passes the FCD check.
          pos = iter.getIndex();
          // Collect the characters being checked, in case they need to be normalized.
          if(s == null) {
              s = new StringBuilder();
          } else {
              s.setLength(0);
          }
          int prevCC = 0;
          for(;;) {
              // Fetch the next character and its fcd16 value.
              int c = iter.nextCodePoint();
              if(c < 0) { break; }
              int fcd16 = nfcImpl.getFCD16(c);
              int leadCC = fcd16 >> 8;
              if(leadCC == 0 && s.length() != 0) {
                  // FCD boundary before this character.
                  iter.previousCodePoint();
                  break;
              }
              s.appendCodePoint(c);
              if(leadCC != 0 && (prevCC > leadCC || CollationFCD.isFCD16OfTibetanCompositeVowel(fcd16))) {
                  // Fails FCD check. Find the next FCD boundary and normalize.
                  for(;;) {
                      c = iter.nextCodePoint();
                      if(c < 0) { break; }
                      if(nfcImpl.getFCD16(c) <= 0xff) {
                          iter.previousCodePoint();
                          break;
                      }
                      s.appendCodePoint(c);
                  }
                  normalize(s);
                  start = pos;
                  limit = pos + s.length();
                  state = State.IN_NORM_ITER_AT_LIMIT;
                  pos = 0;
                  return true;
              }
              prevCC = fcd16 & 0xff;
              if(prevCC == 0) {
                  // FCD boundary after the last character.
                  break;
              }
          }
          limit = pos + s.length();
          assert(pos != limit);
          iter.moveIndex(-s.length());
          state = State.ITER_IN_FCD_SEGMENT;
          return true;
      }
docs: Extends the FCD text segment forward or normalizes around pos.
  @return true if success