XSLT - How to use group by in this situation?

(A follow up of this thread)

I now actually need to know the amount of visiters grouped by Day and then by period. I tried to apply the group-by suggested by Kirill Polishchuk, but I can only get it to group by one certain element. Now I actually need to know how to apply multiple group-by's? E.g. Group by 'Day' first and then by period.

I have the following XML:

<Cinema>
    <Day date="15-09-2011" day="Thursday" week="37" in="543" out="543">
        <Movie name="movie1" in="191" out="191">
            <Period time="16:00:00" in="20" out="20"/>
            <Period time="18:00:00" in="71" out="70"/>
            <Period time="20:00:00" in="100" out="101"/>
        </Movie>
        <Movie name="movie2" in="105" out="105">
            <Period time="16:00:00" in="13" out="13"/>
            <Period time="18:00:00" in="34" out="34"/>
   开发者_JAVA技巧         <Period time="20:00:00" in="58" out="58"/>
        </Movie>
        <Movie name="movie3" in="247" out="247">
            <Period time="16:00:00" in="57" out="57"/>
            <Period time="18:00:00" in="75" out="72"/>
            <Period time="20:00:00" in="115" out="118"/>
        </Movie>
    <Day>
    <Day date="16-09-2011" day="Friday" week="37" in="1151" out="1151">
        <Movie name="movie1" in="364" out="364">
            <Period time="16:00:00" in="106" out="106"/>
            <Period time="18:00:00" in="131" out="129"/>
            <Period time="20:00:00" in="127" out="129"/>
        </Movie>
        <Movie name="movie2" in="333" out="333">
            <Period time="16:00:00" in="89" out="89"/>
            <Period time="18:00:00" in="116" out="116"/>
            <Period time="20:00:00" in="128" out="128"/>
        </Movie>
        <Movie name="movie3" in="454" out="454">
            <Period time="16:00:00" in="104" out="104"/>
            <Period time="18:00:00" in="150" out="150"/>
            <Period time="20:00:00" in="200" out="200"/>
        </Movie>
    <Day>
</Cinema>

What I am trying to get is the total visitors of each movie period per day. For example:

Thursday:
16:00h - in: 90, out: 90
18:00h - in: 180, out: 176
20:00h - in: 273, out: 277
Total - in: 543, out: 543

Friday: 
16:00h - in: 299, out: 299
18:00h - in: 397, out: 395
20:00h - in: 455, out: 457
Total - in: 1151, out: 1151

Does anyone have any suggestions? Thanks in advance!

(I use xslt 1.0 at the moment, but it is possible for me to "upgrade" to the newer 2.0, if necessary)

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output omit-xml-declaration="yes" indent="yes"/>

    <xsl:key name="k" match="Period" use="concat(../../@date, @time)"/>

    <xsl:template match="/Cinema">
        <xsl:apply-templates select="Day"/>
    </xsl:template>

    <xsl:template match="Day">
        <xsl:value-of select="concat(@day, ':')"/>
        <xsl:text>&#xA;</xsl:text>

        <xsl:apply-templates select="*/Period[generate-id(.) = generate-id(key('k', concat(../../@date, @time)))]"/>

        <xsl:value-of select="concat('Total - in: ', @in, ', out: ', @out)"/>
        <xsl:text>&#xA;&#xA;</xsl:text>
    </xsl:template>

    <xsl:template match="Period">
        <xsl:value-of select="
                      concat(substring(@time, 1, 5), 'h - in: ', 
                        sum(key('k', concat(../../@date, @time))/@in), 
                        ', out: ', 
                        sum(key('k', concat(../../@date, @time))/@out))"/>
        <xsl:text>&#xA;</xsl:text>
    </xsl:template>

</xsl:stylesheet>

XML:

<Cinema>
    <Day date="15-09-2011" day="Thursday" week="37" in="543" out="543">
        <Movie name="movie1" in="191" out="191">
            <Period time="16:00:00" in="20" out="20"/>
            <Period time="18:00:00" in="71" out="70"/>
            <Period time="20:00:00" in="100" out="101"/>
        </Movie>
        <Movie name="movie2" in="105" out="105">
            <Period time="16:00:00" in="13" out="13"/>
            <Period time="18:00:00" in="34" out="34"/>
            <Period time="20:00:00" in="58" out="58"/>
        </Movie>
        <Movie name="movie3" in="247" out="247">
            <Period time="16:00:00" in="57" out="57"/>
            <Period time="18:00:00" in="75" out="72"/>
            <Period time="20:00:00" in="115" out="118"/>
        </Movie>
    </Day>
    <Day date="16-09-2011" day="Friday" week="37" in="1151" out="1151">
        <Movie name="movie1" in="364" out="364">
            <Period time="16:00:00" in="106" out="106"/>
            <Period time="18:00:00" in="131" out="129"/>
            <Period time="20:00:00" in="127" out="129"/>
        </Movie>
        <Movie name="movie2" in="333" out="333">
            <Period time="16:00:00" in="89" out="89"/>
            <Period time="18:00:00" in="116" out="116"/>
            <Period time="20:00:00" in="128" out="128"/>
        </Movie>
        <Movie name="movie3" in="454" out="454">
            <Period time="16:00:00" in="104" out="104"/>
            <Period time="18:00:00" in="150" out="150"/>
            <Period time="20:00:00" in="200" out="200"/>
        </Movie>
    </Day>
</Cinema>

Output:

Thursday:
16:00h - in: 90, out: 90
18:00h - in: 180, out: 176
20:00h - in: 273, out: 277
Total - in: 543, out: 543

Friday:
16:00h - in: 299, out: 299
18:00h - in: 397, out: 395
20:00h - in: 455, out: 457
Total - in: 1151, out: 1151