Ah, I messed up, UndefinedSorter must not be chained, it must be a wrapper!

Otherwise comparing to nil will resort to sorting by properties and fail...

������ SortFunction>>undefinedFirst

�������������� ^UndefinedSorter descending wrap: self

������ UndefinedSorter>> collate: value1 with: value2

������������ "sort all nil according to the direction (first if -1, last if +1), then"

�������������� value1 ifNil: [value2 ifNil: [^0] ifNotNil: [^direction]].

�������������� value2 ifNil: [^direction negated].

�������������� ^sorterForNonNil collate: value1 with: value2

It's important to have the UndefinedSorter :

������ - decoupled from property sort, because it can be generally usefull

������ - collating 2 nil as 0, so that another property can be chained

In

������ people sortBy: #name ascending undefinedFirst , #age descending

we could then have people with name nil still sorted by age, what is not possible with current implementation