Debug help - Programming in D - extending tail slice terminates sharing unexpectedly. Page 69, 70

[Brother Bill]

What will cause termination or breakage of tail slice sharing?
1. It is clear that if capacity is 0, then expansion of that 
slice will break sharing.
2. It is explained in Programming in D, that if length < 
capacity, then one can append or extend the length by up to 
(capacity - length), and sharing should continue.
This is what I attempted to do, but failed.

Please guide me to best practices with slice sharing.