In my first attempt, I had written a couple of templates to "hash" or "digitize" a string. I named them after the functions in Zhanyong Wan's Haskell code:

<xsl:template name="charHash">
  <!-- converts a character to its corresponding digit in the keypad -->
  <xsl:param name="ch"/>
  <xsl:value-of select="substring($keypad/key[contains(., $ch)]/., 1, 1)"/>
</xsl:template>

<xsl:template name="strHash">
  <!-- converts a string to its corresponding digit sequence -->
  <xsl:param name="str"/>
  <xsl:param name="i" select="1"/>
  <xsl:call-template name="charHash">
    <xsl:with-param name="ch" select="substring($str, $i, 1)"/>
  </xsl:call-template>
  <xsl:if test="$i < string-length($str)">
    <xsl:call-template name="strHash">
      <xsl:with-param name="str" select="$str"/>
      <xsl:with-param name="i" select="$i + 1"/>
    </xsl:call-template>
  </xsl:if>
</xsl:template>

The $keypad in the "charHash" template pointed to a keypad element in my input file:

<keypad>
  <key>2abc</key>
  <key>3def</key>
  <key>4ghi</key>
  <key>5jkl</key>
  <key>6mno</key>
  <key>7pqrs</key>
  <key>8tuv</key>
  <key>9wxyz</key>
</keypad>

At first, I was satisfied with the code, because it worked, exactly as planned. Until I figured that I could also use XSLT's built-in translate function:

translate($string, 'abcdefghijklmnopqrstuvwxyz', '22233344455566677778889999')

One (simple and clear) line of code replaced the two templates and keypad element. It does exactly the same thing.