problem endless rekursion

hallo,
can you please help me with the following problem.
the problem is to check a check box question. the correct cells and the
false cells should be added. the result are the correct cells. there is a 
endless
rekursion?????

xml part
<CheckBoxQuestion>
  <notice>in Europa kennen wir uns aus!</notice>
  <question>Welche der nachfolgenden Länder gehören zur EU?</question>
  <option number="1">
    <text>Deutschland</text>
    <solution type="bool" points="4" valid_input="1"></solution>
  </option>
  <option number="2">
    <text>Schweiz</text>
    <solution type="bool" points="5" valid_input="0"></solution>
  </option>
  <option number="3">
    <text>Österreich</text>
    <solution type="bool" points="6" valid_input="1"></solution>
  </option>
  <option number="4">
    <text>Türkei</text>
    <solution type="bool" points="7" valid_input="0"></solution>
  </option>
</CheckBoxQuestion>

xsl part
<xsl:template match="CheckBoxQuestion" mode="table">
  <xsl:apply-templates select="option" mode="check" />
</xsl:template>


<xsl:template match="option" mode="check">
  <xsl:variable name="correct_cells" select=".
|following-sibling::solution[valid_input = 1]" />
  <xsl:apply-templates select="$correct_cells" mode="check" />

  <xsl:variable name="false_cells" select=".
|following-sibling::solution[valid_input = 0]" />
  <xsl:apply-templates select="$false_cells" mode="check" />

    <xsl:variable name="correct" select="count($correct_cells) -
count($false_cells)" />

    <xsl:choose>
      <xsl:when test="$correct &gt;= 0">
        <td><xsl:value-of select="$correct" /></td>
      </xsl:when>
      <xsl:otherwise>
        <td>0</td>
      </xsl:otherwise>
    </xsl:choose>

</xsl:template>

<xsl:template match="solution" mode="check">
</xsl:template>

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