newCursorAtValue(ctx context.Context, seq orderedSequence, val Value, forInsertion bool, last bool)
: (*sequenceCursor, error)
newCursorAt(ctx context.Context, seq orderedSequence, key orderedKey, forInsertion bool, last bool)
: (*sequenceCursor, error)
newCursorBackFromValue(ctx context.Context, seq orderedSequence, val Value)
: (*sequenceCursor, error)