code: public static Predicate<Read> getStrictReadPredicate(final long start, final String fields) {
      Preconditions
      .checkArgument(Strings.isNullOrEmpty(fields)
          || READ_FIELD_PATTERN.matcher(fields).matches(),
          "Insufficient fields requested in partial response. At a minimum "
              + "include 'alignments(alignment)' to enforce a strict shard boundary.");
      return new Predicate<Read>() {
        @Override
        public boolean apply(Read read) {
          return read.getAlignment().getPosition().getPosition() >= start;
        }
      };
    }
docs: Predicate expressing the logic for which reads should and should not be included in the shard.
  
  @param start The start position of the shard.
  @return Whether the read would be included in a strict shard boundary.