code: @Nonnull
    @Nonempty
    public static String getXML10EntityReferenceString (final char c)
    {
      if (c == LT)
        return "&lt;";
      if (c == GT)
        return "&gt;";
      if (c == AMPERSAND)
        return "&amp;";
      if (c == DOUBLE_QUOTE)
        return "&quot;";
      if (c == APOS)
        return "&apos;";
      return getXMLNumericReference (c);
    }
docs: Get the entity reference for the specified character. This returns e.g.
  &amp;lt; for '&lt;' etc. This method has special handling for &lt;, &gt;,
  &amp;, &quot; and '. All other chars are encoded by their numeric value (e.g.
  &amp;#200;)
  
  @param c
  Character to use.
  @return The entity reference string. Never <code>null</code> nor empty.