code: public void addProperty(String propertyName, Direction direction) {
          if (propertyName == null) {
              throw new IllegalArgumentException();
          }
          if (direction == null) {
              direction = Direction.UNSPECIFIED;
          }
  
          if (direction != Direction.UNSPECIFIED) {
              if (propertyName.length() > 0) {
                  if (propertyName.charAt(0) == '-' || propertyName.charAt(0) == '+') {
                      // Overrule the direction.
                      propertyName = propertyName.substring(1);
                  }
              }
              propertyName = direction.toCharacter() + propertyName;
          }
  
          mPropertyList.add(propertyName);
      }
docs: Adds a property to this index, with the specified direction.
  
  @param propertyName name of property to add to index
  @param direction optional direction of property