code: @Nonnull
    @Nonempty
    public static String getHTMLEntityReferenceString (final char c)
    {
      if (c == LT)
        return "&lt;";
      if (c == GT)
        return "&gt;";
      if (c == AMPERSAND)
        return "&amp;";
      if (c == DOUBLE_QUOTE)
        return "&quot;";
      // Use of &apos; in XHTML should generally be avoided for compatibility
      // reasons. &#39; or &#x0027; may be used instead.
      return "&#" + (int) c + ";";
    }
docs: Get the entity reference for the specified character. This returns e.g.
  <code>&amp;lt;</code> for '<code>&lt;</code>' etc. This method has special
  handling for &lt;, &gt;, &amp; and &quot;. All other chars are encoded by
  their numeric value (e.g. <code>&amp;#200;</code>). In contrast to
  {@link #getXML10EntityReferenceString(char)} this method does not handle
  <code>&amp;apos;</code>
  
  @param c
  Character to use.
  @return The entity reference string. Never <code>null</code> nor empty.