6 2 9 3 1
 p
6 2 9 3 1
   p
2 6 9 3 1
   p
2 6 9 3 1  increment pointer, trickle down
     p 
2 6 9 3 1  increment pointer, trickle down
       p 
2 6 3 9 1  increment pointer, trickle down
       p
2 3 6 9 1  increment pointer, trickle down
       p
2 3 6 9 1  increment pointer, trickle down
         p
         (several steps
1 2 3 6 9   increment pointer, trickle down
         p 

Mergesort:
    step 1: write a function that zippers two sorted lists into one.
    You will issue forth with profanity several times in this process
    recursive
    Base case:  all lists of length 0 or 1 are sorted!

Pivoting:

[4, 7, 9, 3, -1, 0]
Objective:  first entry is the pivot entry
In-place massge this list so that all of the entries 
smaller than the pivot go on its left and all the bigger
ones go on the right.