how to use ELEMENT: xsl:key?

HI all,

I should to generate this xml file:

<document>
    <line id="0">
        <field id="0"><![CDATA[MAR5555]]></field>
        <field id="1"><![CDATA[something]]></field>
        <field id="2"><![CDATA[something]]></field>
        <field id="3"><![CDATA[something12123]]></field>
        <field id="4"/>
        <field id="5"/>
        <field id="6"/>
        <field id="7"/>
        <field id="8"/>
        <field id="9"/>
        <field id="10"/>
        <field id="11"/>
        <field id="12"/>
        <field id="13"/>
        <field id="14"/>
        <field id="15"/>
        <field id="16"/>
        <field id="17"><![CDATA[0072972+1313113123123]]></field>
        <field id="18"><![CDATA[5353]]></field>
        <field id="19"><![CDATA[444432323]]></field>
        <field id="20"/>
        <field id="21"/>
    </line>
    <line id="1">
        <field id="0"><![CDATA[MAR6435]]></field>
        <field id="1"><![CDATA[car123]]></field>
        <field id="2"><![CDATA[sds]]></field>
        <field id="3"><![CDATA[fest]]></field>
        <field id="4"/>
        <field id="5"/>
        <field id="6"/>
        <field id="7"/>
        <field id="8"/>
        <field id="9"/>
        <field id="10"/>
        <field id="11"/>
        <field id="12"/>
        <field id="13"/>
        <field id="14"/>
        <field id="15"><![CDATA[0000062+0dadasd]]></field>
        <field id="16"><![CDATA[032]]></field>
        <field id="17"><![CDATA[23242442]]></field>
        <field id="18"/>
        <field id="19"><![CDATA[000000]]></field>
        <field id="20"/>
        <field id="21"/>
    </line>
    <line id="2">
        <field id="0"><![CDATA[MAR6435]]></field>
        <field id="1"><![CDATA[sss]]></field>
        <field id="2"><![CDATA[Something111]]></field>
        <field id="3"><![CDATA[something111]]></field>
        <field id="4"/>
        <field id="5"/>
        <field id="6"/>
        <field id="7"/>
        <field id="8"/>
        <field id="9"/>
        <field id="10"/>
        <field id="11"/>
        <field id="12"/>
        <field id="13"/>
        <field id="14"/>
        <field id="15"><![CDATA[0000062+0sdsd]]></field>
        <field id="16"><![CDATA[022]]>开发者_如何学JAVA;</field>
        <field id="17"><![CDATA[23444444]]></field>
        <field id="18"/>
        <field id="19"><![CDATA[000000]]></field>
        <field id="20"/>
        <field id="21"/>
    </line>
</document>

My programs looks:

<xsl:stylesheet version="1.0"xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes" omit-xml-declaration="yes"/>
<xsl:key name="kLine" match="line" use="substring-before(field[contains(., '+')],'+')"/>
    <xsl:template match="/*">
        <document>
         <xsl:apply-templates select="line[contains(field[@id='0'], 'MAR')][count( . | key('kLine',substring-before(field[contains(., '+')],'+'))[1]) = 1]"/>
        </document>
    </xsl:template>
    <xsl:template match="line">
      <type-MAR>
       <document>
        <xsl:value-of select="substring-before(field[contains(., '+')],'+')"/> 
       </document>
        <!--     For each document-->
        <line>
         <LineNumber>
          <xsl:value-of select="position()"/>
         </LineNumber>
         <LineItem>
           <xsl:value-of select="'should be fielled number of after two fields which contain symbol + '"/>
         </LineItem>
        </line>
        <!-- For each document-->
       </type-MAR>
        </xsl:template>
</xsl:stylesheet>

I'm stuck, taking LineItem number after field id which contains symbol '+'. for example if I know that 'field id = 17' has + symbol, when after two rows I will have LineItem and do calculating I mean if 'field id = 17' when 'field id = 17+2', if 'field id = 16' then 'field id = 18' and so on. but now when field with symbol '+' are not defining, I can't to do it. Also if I use 'key', i want to ask how to use correct cycle "for each", to calculate number of lines. Result should be:

<document>
    <type-MAR>
        <document>0072972</document>
        <line>
            <LineNumber>1</LineNumber>
            <LineItem>444432323</LineItem>
        </line>
    </type-MAR>
    <type-MAR>
        <document>0000062</document>
        <line>
            <LineNumber>1</LineNumber>
            <LineItem>23242442</LineItem>
            <LineNumber>2</LineNumber>
            <LineItem>23444444</LineItem>
        </line>
    </type-MAR>
</document>

I solve it by self. maybe my question was not clearly. first off all to found path to field after document number I use Following sibling. to select all needed LineItem I use "variable name", finally program looks:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output method="xml" indent="yes" omit-xml-declaration="yes"/>
    <xsl:key name="kLine" match="line" use="substring-before(field[contains(., '+')],'+')"/>
    <xsl:template match="/*">
        <document>
            <xsl:apply-templates select="line[contains(field[@id='0'], 'MAR')][count( . | key('kLine',substring-before(field[contains(., '+')],'+'))[1]) = 1]"/>
        </document>
    </xsl:template>
    <xsl:template match="line">
    <xsl:variable name="doc_code" select="substring-before(field[contains(., '+')],'+')"/>
        <type-MAR>
            <document>
                <xsl:value-of select="substring-before(field[contains(., '+')],'+')"/>
            </document>
            <xsl:for-each select="//line[substring-before(field[contains(., '+')],'+')=$doc_code]">
                            <line>
                    <LineNumber>
                        <xsl:value-of select="position()"/>
                    </LineNumber>
                    <LineItem>
                    <xsl:value-of select="field[contains(., '+')]/following-sibling::field[2]"/>
                    </LineItem>
                </line>
            </xsl:for-each>
        </type-MAR>
    </xsl:template>
</xsl:stylesheet>

Best regards