Yes jbrugger, It should be there. (I'll try to draw a diagram for that). I hope this drawing won't complicate the intention:

set1' => {
           'key1_set1' => [ 'some arrays' ], --->-
           'key2_set1' => [ 'some arrays' ], +++>|+++>+
         },                                      |    +
                                                 |    +
'set2' => {                                      |    +
           'key1_set2' => [ 'some arrays' ], <---|<+++>
           'key2_set2' => [ 'some arrays' ], <---|<+++>
         },                                      |    +
                                                 |    +   etc...until
'set3' => {                                      |    +
           'key1_set3' => [ 'some arrays' ], <---|<++++ ###>
           'key2_set3' => [ 'some arrays' ], <---|<++++ ###>
         },                                      |    +    #
                                                 |    +    #
'set4' => {                                      |    +    #
           'key1_set4' => [ 'some arrays' ], <---|<++++ <###
           'key2_set4' => [ 'some arrays' ], <---|<++++ <###
           'key3_set4' => [ 'some arrays' ], <---|<++++ <###
         },
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