Add Procedures Vet and Draw
Implement Link

1. Use Protected Linker to link, for testing, so far.
2. Separate function Activate to attach a 'removed node'
3. Add Labels, Comments and Asserts
4. Add procedure  Vet and Drow
Bugfix

Avoid Duplicated Element insertion
Update

1. Move exchange operations closer
2. More Assertions
3. Improve procedure Draw