Hi,
Using 8.2.3 (and the latest beta) this code seems to be invoking the JSON serializer. I don't understand why. 

declare function local:wrap2($doc,$x as xs:integer*){
 $doc
};
let $b:=doc("file.xml")
return array:for-each([1],local:wrap2($b,?))

And if file.xml is
<doc>
    <p style="prior"/>
    <p style="target"/>
    <p style="later"/>
</doc>

Error:
Improper use? Potential bug? Your feedback is welcome:
Contact: basex-talk@mailman.uni-konstanz.de
Version: BaseX 8.3 beta 89201d1
Java: Oracle Corporation, 1.8.0_45
OS: Windows 7, amd64
Stack Trace: 
java.lang.NullPointerException
at org.basex.util.Token.eq(Token.java:619)
at org.basex.io.serial.json.JsonNodeSerializer.finishOpen(JsonNodeSerializer.java:145)
at org.basex.io.serial.Serializer.prepare(Serializer.java:524)
at org.basex.io.serial.Serializer.openElement(Serializer.java:229)
at org.basex.io.serial.Serializer.node(Serializer.java:437)
at org.basex.io.serial.Serializer.node(Serializer.java:176)
at org.basex.io.serial.StandardSerializer.node(StandardSerializer.java:99)
at org.basex.io.serial.json.JsonNodeSerializer.node(JsonNodeSerializer.java:89)
at org.basex.io.serial.Serializer.serialize(Serializer.java:126)
at org.basex.io.serial.StandardSerializer.serialize(StandardSerializer.java:84)
at org.basex.io.serial.json.JsonSerializer.serialize(JsonSerializer.java:57)
at org.basex.io.serial.json.JsonSerializer.serialize(JsonSerializer.java:76)
at org.basex.io.serial.json.JsonSerializer.function(JsonSerializer.java:120)
at org.basex.io.serial.AdaptiveSerializer.function(AdaptiveSerializer.java:78)
at org.basex.io.serial.Serializer.serialize(Serializer.java:128)
at org.basex.io.serial.AdaptiveSerializer.serialize(AdaptiveSerializer.java:63)
at org.basex.query.value.Value.serialize(Value.java:223)
at org.basex.core.cmd.AQuery.query(AQuery.java:85)
at org.basex.core.cmd.XQuery.run(XQuery.java:22)
at org.basex.core.Command.run(Command.java:398)
at org.basex.core.Command.execute(Command.java:100)
at org.basex.gui.GUI.exec(GUI.java:472)
at org.basex.gui.GUI.access$4(GUI.java:425)
at org.basex.gui.GUI$7.run(GUI.java:412)