code: @Nonnull
    public CSSExpression addString (@Nonnegative final int nIndex, @Nonnull final String sValue)
    {
      return addTermSimple (nIndex, getQuotedStringValue (sValue));
    }
docs: Shortcut method to add a string value that is automatically quoted inside
  
  @param nIndex
  The index where the member should be added. Must be &ge; 0.
  @param sValue
  The value to be quoted and than added. May not be <code>null</code>.
  @return this