<xsl:variable
name="stopPat">it|its|itself|they|them|their|what|which|who|whom|this|that|these|those|am|is|are|was|were|be|been|being|have|has|had|having|do|does|did|doing|a|an|the|and|but|if|or|because|as|until|while|of|at|by|for|with|about|against|between|into|through|during|before|after|above|below|to|from|up|down|in|out|on|off|over|under|again|further|then|once|here|there|when|where|why|how|all|any|both|each|few|more|most|other|some|such|no|nor|not|only|own|same|so|than|too|very|s|t|can|will|just|don|should|now</xsl:variable>

<xsl:variable name="stops" select="tokenize($stopPat,'\|')"/>

  <xsl:function name="my:stop1" as="xs:boolean">
   <xsl:param name="w" as="xs:string"/>
   <xsl:sequence select="some $s in $stops satisfies ($s eq $w)"/>
  </xsl:function>

For obvious reasons this seems unlikely to be very efficient.