Compat.SeqIf xs is empty, then uncons xs is None.
If xs is nonempty, then uncons xs is Some (x, ys) where x is the head of the sequence and ys its tail.
is_empty xs determines whether the sequence xs is empty.
It is recommended that the sequence xs be persistent. Indeed, is_empty xs demands the head of the sequence xs, so, if xs is ephemeral, it may be the case that xs cannot be used any more after this call has taken place.
append xs ys is the concatenation of the sequences xs and ys.
Its elements are the elements of xs, followed by the elements of ys.