XSLT for each letter in a string

I'm writing an XSLT transformation (for XSL-FO), and need to repeat something for each letter in a string value, for example:

If string is stored in MyData/MyValue string (e.g. MyData.MyValue = "something"), I need an for-each like this one:

<xsl:for-each select="MyData/MyValue"> <!-- What goes here to iterate through letters? -->
  <someTags>
    <xsl:value-of select="Letter" /> <!-- What goes here to output current letter? -->
  </someTags>
</xsl:for-each>

Any ideas?