Functionality Implementation
| Functionality |
| The `<xsl:comment>` element is used to insert XML comments into the result document. The comment is inserted as follows in the XSL stylesheet:
        <element>
          <xsl:comment>This is a comment!</xsl:comment>
        </element>
and it will be output in the result document as follows:
        <element>
          <!-- This is a comment! -->
        </element>
The `<xsl:message>` element is used to send messages to who/whatever is performing the transformation. The message can be displayed in a terminal, a dialog box, etc. The `<xsl:message>` element is used in the same way as the `<xsl:comment>` element, however the message is not output in the result document. The `<xsl:message>` element has an attribute `"terminate"` which can be used to terminate the transformation at any given stage. This is useful when using message to give information about illegal values in the input document.
| Implementation |
| The output handler class has a method for outputting comments in the output document and messages to the operator. The code compiled for a comment gathers all text that goes into the comment in a StringValueHandler object, retrieves the full text string from this handler, and then finally sends it to the output handler. Similarly the code compiled for a message will simply send the message to the output handler in use. Messages will be output to the terminal (stdout) when a transformation is run in a terminal. The message will be output before the beginning of the output document. Messages will be shown in a dialog box if a transformation is run in an applet. `<xsl:message>` elements that use the `"terminate"` attribute to abort transformations cause an exception to be thrown. A RuntimeException is used for this, and the exception text is: `Termination forced by an xsl:message instruction`

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