LList.add(PositionContainer posSet, int posNumber, Object value)

Does SeqIterator.remove work for SimpleVector, GapVector, StableVector?

TreeList should extend AbstractSequence and implement Sequence.

Should U64Vector (etc) extend S64Vector (etc)?  Would share some code,
but I don't think a U64Vector "is-a" S64Vector.

PositionSet class (subsumes DOM NodeSet).

Old CharBuffer had replace(int where, char[] chars, int start, int count),
and replace(int where, String string).

Old AbstractString had InPort open(), and replace(int where, char[]
chars, int start, int count), and replace(int where, String string),
and replace (int where, AbstractString string) and FString copy(), and
insert(int where, int count, boolean beforeMarkers) and
insert(int where, AbstractString str, boolean beforeMarkers) and
insert(int where, char[] chars, int start, int count, boolean beforeMarkers)
and insert(int where, char ch, int count, boolean beforeMarkers).

class SubString extends SubSequence implements CharSequence.
public SubString(CharSequence base, ...) { ... }
