Xquery evaluation

Question

I'm trying to execute the xquery function 'GuidsReferencingTo' (code is below) and getting this error ''Error: It is an error for the context item to be undefined when using it [err:XPDY0002], <query>:42:58'. The error happens in the function 'GuidChildren' that is called from "where" block (via SelfAndDerivedGuids and DerivedGuids functions). First I would expect ‘let’ clause to be evaluated before, but it looks like it's being evaluated in ‘where’ clause (does it happens in each loop?). Second, it looks like somewhere empty sequence causes this issue somehow. Disabling both 'some' clauses eliminates the issue.
There no problems when 'GuidChildren', ‘DerivedGuids’ or ‘SelfAndDerivedGuids’ are not involved into ‘some’ clause (they used in many places).
declare function pg:SelectByID ( $id as xs:string )
let $t := dbxml:lookup-attribute-index ( "D:/Projects/adb/xml_data/data.dbxml", "pg:id" )[.=$id]/parent::*
(:let $t := collection ( "dbxml:/D:/Projects/adb/xml_data/data.dbxml" )//pg:*[@pg:id=$id]:)
     return
     if ( fn:empty ( $t ) )
then pg:SelectByID ( '00000000000000000000000000000000' )
else $t
declare function pg:Self ( $id as xs:string )
let $r := pg:SelectByID ( $id )
return
if ( $r/@pg:type = "00000000000000000000000000000007" )
then pg:SelectByID ( $r/@pg:parent cast as xs:string )
else $r
declare function pg:GuidChildren ( $id as xs:string )
(:dbxml:lookup-attribute-index ( 'c:/adb/data/data.dbxml', 'pg:parent' )[.=$id]/parent::*:)
collection ( "dbxml:/c:/adb/data/data.dbxml" )//pg:*[@pg:parent=$id]
declare function pg:DerivedGuids ( $id as xs:string )
for $t in pg:GuidChildren ( $id )
return ( $t, pg:DerivedGuids ( $t/@pg:id cast as xs:string ) )
declare function pg:SelfAndDerivedGuids ( $id as xs:string )
let $t := pg:Self ( $id )
return
if ( $t/@pg:id != '00000000000000000000000000000000' )
then ( $t, pg:DerivedGuids ( $t/@pg:id cast as xs:string ) )
else $t
declare function pg:GuidsReferencingTo ( $type as xs:string, $attrRef as xs:string, $attrType as xs:string )
let $t := pg:SelfAndDerivedGuids ( $type )
let $rt := pg:SelfAndDerivedGuids ( $attrType )
for $r in dbxml:lookup-attribute-index ( 'c:/adb/data/data.dbxml', 'pg:ref' )[.=$attrRef]/parent::pg:r
where
( some $n in $rt satisfies $r/@pg:rt = $n/@pg:id ) and
( some $m in $t satisfies $r/parent::*/@pg:type = $m/@pg:id )
return $r/parent::*
};

Answer 1

formatted code
declare function pg:SelectByID ( $id as xs:string )
    let $t := dbxml:lookup-attribute-index ( "D:/Projects/adb/xml_data/data.dbxml", "pg:id" )[.=$id]/parent::*
    (:let $t := collection ( "dbxml:/D:/Projects/adb/xml_data/data.dbxml" )//pg:*[@pg:id=$id]:)
     return
         if ( fn:empty ( $t ) )
            then pg:SelectByID ( '00000000000000000000000000000000' )
            else $t
declare function pg:Self ( $id as xs:string )
    let $r := pg:SelectByID ( $id )
    return
        if ( $r/@pg:type = "00000000000000000000000000000007" )
            then pg:SelectByID ( $r/@pg:parent cast as xs:string )
            else $r
declare function pg:GuidChildren ( $id as xs:string )
    (:dbxml:lookup-attribute-index ( 'c:/adb/data/data.dbxml', 'pg:parent' )[.=$id]/parent::*:)
    collection ( "dbxml:/c:/adb/data/data.dbxml" )//pg:*[@pg:parent=$id]
declare function pg:DerivedGuids ( $id as xs:string )
    for $t in pg:GuidChildren ( $id )
    return ( $t, pg:DerivedGuids ( $t/@pg:id cast as xs:string ) )
declare function pg:SelfAndDerivedGuids ( $id as xs:string )
    let $t := pg:Self ( $id )
    return
        if ( $t/@pg:id != '00000000000000000000000000000000' )
            then ( $t, pg:DerivedGuids ( $t/@pg:id cast as xs:string ) )
            else $t
declare function pg:GuidsReferencingTo ( $type as xs:string, $attrRef as xs:string, $attrType as xs:string )
    let $t := pg:SelfAndDerivedGuids ( $type )
    let $rt := pg:SelfAndDerivedGuids ( $attrType )
    for $r in dbxml:lookup-attribute-index ( 'c:/adb/data/data.dbxml', 'pg:ref' )[.=$attrRef]/parent::pg:r
        where
            ( some $n in $rt satisfies $r/@pg:rt = $n/@pg:id ) and
            ( some $m in $t satisfies $r/parent::*/@pg:type = $m/@pg:id )
        return $r/parent::*
};