Trying to understand XSLT Muenchian Method transformation - xslt

Question

I came across the Muenchian Method when looking for a way of grouping elements in an XML file that has been produced by converted a CSV file.
Source
<file>
<patient>
<Lab_Specimen_Number>L,18.1342718.Y</Lab_Specimen_Number>
<Patient_Number>LOC0000015</Patient_Number>
<ORG/>
<Specimen/>
<Antibiotic_Amox_Ampicillin/>
</patient>
<patient>
<Lab_Specimen_Number>L,18.1342727.V</Lab_Specimen_Number>
<Patient_Number>LOC0000001</Patient_Number>
<ORG>Coliform</ORG>
<Specimen>L,18.1342727.VA</Specimen>
<Antibiotic_Amox_Ampicillin>S</Antibiotic_Amox_Ampicillin>
</patient>
<patient>
<Lab_Specimen_Number/>
<Patient_Number/>
<ORG>Staphylococcus aureus</ORG>
<Specimen>L,18.1342727.VA</Specimen>
<Antibiotic_Amox_Ampicillin>S</Antibiotic_Amox_Ampicillin>
</patient>
<patient>
<Lab_Specimen_Number>L,18.1346290.T</Lab_Specimen_Number>
<Patient_Number>LOC0000001</Patient_Number>
<ORG>Coliform</ORG>
<Specimen>L,18.1346290.TA</Specimen>
<Antibiotic_Amox_Ampicillin>S</Antibiotic_Amox_Ampicillin>
</patient>
<patient>
<Lab_Specimen_Number>L,18.1342713.X</Lab_Specimen_Number>
<Patient_Number>LOC0000009</Patient_Number>
<ORG/>
<Specimen/>
<Antibiotic_Amox_Ampicillin/>
</patient>
</file>
Based on the article I have changed the match when assigning a key to patient[Specimen != ''] instead of just patient as it is possible for the Specimen value to be blank and these would be missing from the final output if just patient was used.
Transformation
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="xml" indent="yes" omit-xml-declaration="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="patients-by-specimen" match="patient[Specimen != '']" use="Specimen" />
<xsl:template match="file">
<file>
<xsl:for-each select="patient[count(. | key('patients-by-specimen', Specimen)[1]) = 1]">
<patient>
<xsl:copy-of select="Lab_Specimen_Number" />
<xsl:copy-of select="Patient_Number" />
<Specimen>
<xsl:copy-of select="Specimen" />
<Organisms>
<xsl:for-each select="key('patients-by-specimen', Specimen)">
<Organism>
<xsl:copy-of select="ORG"/>
<xsl:copy-of select="Antibiotic_Amox_Ampicillin"/>
</Organism>
</xsl:for-each>
</Organisms>
</Specimen>
</patient>
</xsl:for-each>
</file>
</xsl:template>
</xsl:stylesheet>
Whilst the transformation above gives me the desired output I don't fully understand how this line is working:
<xsl:for-each select="patient[count(. | key('patients-by-specimen', Specimen)[1]) = 1]">
Could someone explain this iteration in the context of my source file?
Output
<file>
<patient>
<Lab_Specimen_Number>L,18.1342718.Y</Lab_Specimen_Number>
<Patient_Number>LOC0000015</Patient_Number>
<Specimen>
<Specimen/>
<Organisms/>
</Specimen>
</patient>
<patient>
<Lab_Specimen_Number>L,18.1342727.V</Lab_Specimen_Number>
<Patient_Number>LOC0000001</Patient_Number>
<Specimen>
<Specimen>L,18.1342727.VA</Specimen>
<Organisms>
<Organism>
<ORG>Coliform</ORG>
<Antibiotic_Amox_Ampicillin>S</Antibiotic_Amox_Ampicillin>
</Organism>
<Organism>
<ORG>Staphylococcus aureus</ORG>
<Antibiotic_Amox_Ampicillin>S</Antibiotic_Amox_Ampicillin>
</Organism>
</Organisms>
</Specimen>
</patient>
<patient>
<Lab_Specimen_Number>L,18.1346290.T</Lab_Specimen_Number>
<Patient_Number>LOC0000001</Patient_Number>
<Specimen>
<Specimen>L,18.1346290.TA</Specimen>
<Organisms>
<Organism>
<ORG>Coliform</ORG>
<Antibiotic_Amox_Ampicillin>S</Antibiotic_Amox_Ampicillin>
</Organism>
</Organisms>
</Specimen>
</patient>
<patient>
<Lab_Specimen_Number>L,18.1342713.X</Lab_Specimen_Number>
<Patient_Number>LOC0000009</Patient_Number>
<Specimen>
<Specimen/>
<Organisms/>
</Specimen>
</patient>
</file>