[xslt] Conditional includes or definitions

["Sam Liddicott" <sam liddicott com>]

<xsl:if test="not(function-available('str:tokenalize'))">
  <func:function ....

would be nice to define alternate implementations for missing functions,
except that <xsl:if can't be in a top level scope like that.

Is there a neat way to do this, or is the simplest way to have a wrapper
function which uses function-available to decide whether to call the
built-in function or the xslt-defined one?

Sam