Extract collection from register

[catena2w]

I can't find an example of collection extraction from a register, and expected way to do this like ExtractRegisterAs[SCollection[Boolean]](Self, R4)(SCollection[Boolean]) does not work

[kushti]

it shouldn't work also, as it is impossible to define collection size (i.e. cost) before transformations

[catena2w]

Some sort of user-defined collections is required. E.g. If we want AVLTreeModification operation, user (not a script) should provide collection of modifications and proof for them.