It would help to know which version of Pharo you are using.

#(1 2 3) flatten

raises an exception in Pharo 9 because there is no #flatten

in Array's protocol.

Collection has

��#flatCollect:

���� -- #gather: was not a terribly helpful name,

���� -- but #flatCollect: is horribly misleading.

��#flatCollect:as:

���� -- A misplaced method, should be

���� -- Collection class >> withAll: aCollection gather: aBlock

��#flatCollectAsSet:

���� -- a trivial special case of #flatCollect:as:,

���� -- not clear why this special case exists.

��#flattenOn:

���� -- aStream nextPutAll: self flattened

���� -- but without creating the intermediate data structure

��#flattened

���� -- a curiously twisted "flatten" that does not work

���� -- at all for non-collections.

#(1 2 3) flattened => #(1 2 3)

1 flattened => BOOM!

By the way, we cannot blame the cruftiness of #flattened on

40 years of history.�� #flatten/#flattened only entered Squeak

about 7 years ago and it was BORN crufty.�� I mistakenly thought

Squeak and Pharo agreed on what it meant.�� They do not.

Squeak: puts it on SequenceableCollection and Stream.

Pharo : puts it on Collection (more general) and not Stream (less so).

Squeak: treats stream elements like nested collections.

Pharo : doesn't.

So my earlier claim bears repeating: of the Smalltalk systems that

have some kind of flattening, no two agree on what it means.

The one use I have ever had for flattening is to build up a

large string by accumulating characters, strings, symbols,

and sequences of the same in a sequence and then flattening

the tree into a single string, which is something that

Squeak and Pharo flatly refuse to do.

One improvement I would like to see in Pharo 11 is for these

methods to be deleted.