Hello Dr. GrĂ¼n,
 
I open the database 'Collection' in the basex tool 
and enter the following query 
 
for $x in collection('Collection')
return $x//PoetSession
 
and get the following error message under Query Info.
If you want I can provide you with the database files.
 
Kind regards,
 
Achim. 
 
 
Error:
Improper use? Potential bug? Your feedback is welcome:
Contact: basex-talk@mailman.uni-konstanz.de
Version: BaseX 7.9
Java: Sun Microsystems Inc., 1.6.0_21
OS: Windows XP, x86
Stack Trace: 
java.lang.RuntimeException: Data Access out of bounds:
- pre value: 75055
- #used blocks: 294
- #total locks: 294
- access: 293 (294 > 293]
        at org.basex.util.Util.notExpected(Util.java:53)
        at org.basex.io.random.TableDiskAccess.cursor(TableDiskAccess.java:508)
        at org.basex.io.random.TableDiskAccess.read1(TableDiskAccess.java:194)
        at org.basex.data.Data.kind(Data.java:284)
        at org.basex.query.value.node.DBNode.<init>(DBNode.java:53)
        at org.basex.query.value.seq.DBNodeSeq.itemAt(DBNodeSeq.java:65)
        at org.basex.query.value.seq.DBNodeSeq.itemAt(DBNodeSeq.java:19)
        at org.basex.query.value.seq.Seq$1.next(Seq.java:92)
        at org.basex.query.gflwor.For$1.next(For.java:71)
        at org.basex.query.gflwor.GFLWOR$2.next(GFLWOR.java:74)
        at org.basex.query.MainModule$1.next(MainModule.java:98)
        at org.basex.query.QueryContext.execute(QueryContext.java:580)
        at org.basex.query.QueryProcessor.execute(QueryProcessor.java:101)
        at org.basex.core.cmd.AQuery.query(AQuery.java:82)
        at org.basex.core.cmd.XQuery.run(XQuery.java:22)
        at org.basex.core.Command.run(Command.java:329)
        at org.basex.core.Command.execute(Command.java:94)
        at org.basex.gui.GUI.exec(GUI.java:417)
        at org.basex.gui.GUI.access$500(GUI.java:41)
        at org.basex.gui.GUI$8.run(GUI.java:361)
Compiling:
- pre-evaluating fn:collection("Collection")
- simplifying descendant-or-self step(s)
Query:
for $x in collection('Collection') return $x//PoetSession
Optimized Query:
for $x_0 in (db:open-pre("Collection",0), db:open-pre("Collection",766), ...) return $x_0/descendant::PoetSession
Query plan:
<QueryPlan>
  <GFLWOR>
    <For>
      <Var name="$x" id="0"/>
      <DBNodeSeq size="103">
        <DBNode name="Collection" pre="0"/>
        <DBNode name="Collection" pre="766"/>
        <DBNode name="Collection" pre="1242"/>
        <DBNode name="Collection" pre="2008"/>
        <DBNode name="Collection" pre="2777"/>
      </DBNodeSeq>
    </For>
    <IterPath>
      <VarRef>
        <Var name="$x" id="0"/>
      </VarRef>
      <IterStep axis="descendant" test="PoetSession"/>
    </IterPath>
  </GFLWOR>
</QueryPlan>