Hello, I made a small library that reify grouping (that one does via #groupBy: usually): Grouper https://github.com/juliendelplanque/Grouper It is designed in the same spirit as SortFunction, but for grouping. Quick example: The following code snippet using Grouper: (10 to: 50)groupUsing: [:integer | integer asString first ]. "an OrderedDictionary( $1->#(10 11 12 13 14 15 16 17 18 19) $2->#(20 21 22 23 24 25 26 27 28 29) $3->#(30 31 32 33 34 35 36 37 38 39) $4->#(40 41 42 43 44 45 46 47 48 49) $5->#(50))" is equivalent to the following code snippet using built-in|#groupedBy:|method: (10 to: 50)groupedBy: [:integer | integer asString first ] "an OrderedDictionary( $1->#(10 11 12 13 14 15 16 17 18 19) $2->#(20 21 22 23 24 25 26 27 28 29) $3->#(30 31 32 33 34 35 36 37 38 39) $4->#(40 41 42 43 44 45 46 47 48 49) $5->#(50))" The advantage of using it is that it allows one to compose grouper objects. Thus, it is easy to describe grouping on 2 or 3 levels. Fore example: The power of Grouper is that group description are first-class objects. Thus, it is possible to compose group descriptions in order to group a flat collection on multiple levels. For example: groupComposition:= [:integer | integer asString first ] grouper , [:integer | integer asString second ]. (10 to: 50)groupUsing: groupComposition. "an OrderedDictionary( $1->an OrderedDictionary( $0->#(10) $1->#(11) $2->#(12) $3->#(13) $4->#(14) $5->#(15) $6->#(16) $7->#(17) $8->#(18) $9->#(19)) $2->an OrderedDictionary( $0->#(20) $1->#(21) $2->#(22) $3->#(23) $4->#(24) $5->#(25) $6->#(26) $7->#(27) $8->#(28) $9->#(29)) $3->an OrderedDictionary( $0->#(30) $1->#(31) $2->#(32) $3->#(33) $4->#(34) $5->#(35) $6->#(36) $7->#(37) $8->#(38) $9->#(39)) $4->an OrderedDictionary( $0->#(40) $1->#(41) $2->#(42) $3->#(43) $4->#(44) $5->#(45) $6->#(46) $7->#(47) $8->#(48) $9->#(49)) $5->an OrderedDictionary($0->#(50)))" On the github, there are more example showing how to build trees with custom objects. Any feedback for this library is welcome. I would like to propose it to Pharo in the near future because it would simplify a lot of code in DrTests (related to result tree views, the tree on the right of the UI). I don't know if it will be possible before next release as we are in feature freeze. Cheers, Julien