Substring value Processing in recursion - xslt

Question

This is all about reverse a string. It works properly for the given Value 'ABCDEF'. The output is also correct 'FEDCBA'. But I want to know how this is printing letters 'A' and 'D' in this string. Could anyone help me to understand this? please.
Elaborate this working method.
<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-result-prefixes="xs">
<xsl:output method="html"/>
<xsl:template name="reverse">
<xsl:param name="input" select="'ABCDEF'"/>
<xsl:variable name="len" select="string-length($input)"/>
<xsl:choose>
<xsl:when test="$len < 2">
<xsl:value-of select="$input"/>
</xsl:when>
<xsl:when test="$len = 2">
<xsl:value-of select="substring($input,2,1)"/>
<xsl:value-of select="substring($input,1,1)"/>
</xsl:when>
<xsl:otherwise>
<xsl:variable name="mid" select="floor($len div 2)"/>
<xsl:call-template name="reverse">
<xsl:with-param name="input" select="substring($input,$mid+1,$mid+1)"/>
</xsl:call-template>
<xsl:call-template name="reverse">
<xsl:with-param name="input" select="substring($input,1,$mid)"/>
</xsl:call-template>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
<xsl:template match="/">
<xsl:call-template name="reverse">
</xsl:call-template>
</xsl:template>
</xsl:stylesheet>

Answer 1

Here's what happens:
Input ABCDEF
len = 6
len is > 2 and not = 2
mid = 3
Call again with input DEF
len = 3
len is > 2 and not = 2
mid = 1
Call again with input EF
len = 2
len is not > 2 but = 2
Put out the second letter F
Put out the first letter E
Return to outer call
Call with input = D
len is less than 2
Output D
Return to outer call
Call again with input = ABC
len = 3
len is > 2 and not 2
mid = 1
Call again with input BC
len = 2
len is not > 2 but = 2
Put out the second letter C
Put out the first letter B
Return to outer call
Call with input A
len is less than 2
Output A