<xsl:choose> / <xsl:when> / <xsl:otherwise>

| Contents |
| Functionality Implementation
| Functionality |
| The `<xsl:choose>` element is used to determine one course of action based on a series of tests. Each test is done inside an `<xsl:when>` element. If a test succeeds, the body of the `<xsl:when>` element is executed. If no tests fail then a `<xsl:otherwise>` element can be used to specify a default action:
        <xsl:choose>
          <xsl:when test="element-available('some-extension')">
              ...
          </xsl:when>
          <xsl:when test="function-availabe('saxon:nodeset')">
              ...
          </xsl:when>
          <xsl:otherwise>
              ...
          </xsl:otherwise>
        </xsl:choose>
| Implementation |
| The `Choose` class places all `When` child-nodes in a vector. The `Choose` class translates the "test"-attribute of all `When` nodes (in strict order) and chains them together in an if-else style. The expression that holds each test contains a true- and a false-list. These lists are vectors of branch targets that should be used if the test succeeds or fails, respectively. The first test's false-list is pointed to the start of the next test (ie. if the first test fails, then we run the next test). The last test's false-list points directly to the code for the body of the `<xsl:otherwise>` element. Just as with the `<xsl:if>`-element, special care is taken for the `element-available()` and `function-available()` functions. These functions are evaluated at compile-time (this can be done since all parameters for these functions are literals) and the body of a `<xsl:when>` element is not compiled if we know that it will never be needed at runtime.

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