Xquery for-if-then-else unexpected output

I am trying to return some output and am I'm having some difficulties, it may very well be I don't have the correct understanding of Xquery for-if-then--else statements.

for $S in doc("prac5a.xml")/enrolment return <enrolment>{for $s in//student, $e in//enrol return  if($s/id = $e/stud) then <student>{$s/id, $s/name, $e/crs}</student> else <student>{$s/id, $s/name}</student>}</enrolment>

From this I was expecting an output of something like;

<enrolment>
    <student>
        <id>s1</id>
        <name>John</name>
        <crs>c1</crs>
        <crs>c2</crs>
    </student>
    <student>
        <id>s2</id>
        <name>Peter</name>
        <crs>c2</crs>
        <crs>c3</crs>
    </student>
    <student>
        <id>s3</id>
        <name>Rob</name>
    </student>

Instead I get this;

<?xml version="1.0" encoding="UTF-8"?>
<enrolment>
   <student>
      <id>s1</id>
      <name>John</name>
      <crs>c1</crs>
      <crs>c2</crs>
   </student>
   <student>
      <id>s1</id>
      <name>John</name>
   </student>
   <student>
      <id>s2</id>
      <name>Peter</name>
   </student>
   <student>
      <id>s2</id>
      <name>Peter</name>
      <crs>c2</crs>
      <crs>c3</crs>
   </student>
   <student>
      <id>s3</id>
      <name>Rob</name>
   </student>
   <student>
      <id>s3</id>
      <name>Rob</name>
   </student>
</enrolment>

Why is that? or more importantly I suppose how could I get the desired result.