gistfile1.txt

LHS | RHS   | Result
----|-------|-------
A   | Bpre  | A
B   | B     | Bpre
C   | Bpost | B
D   | Hpre  | C
E   | H     | D
F   | Hpost | F
G   | Epre  | G
H   | E     | Bpost
I   | Epost | Hpre
J   | L     | H
K   | M     | I
    |       | J
    |       | Hpost
    |       | Epre
    |       | E
    |       | Epost
    |       | L
    |       | M

Start with LHS and RHS

```
LHS | RHS   
----|-------
A   | Bpre  
B   | B     
C   | Bpost 
D   | Hpre  
E   | H     
F   | Hpost 
G   | Epre  
H   | E     
I   | Epost 
J   | L     
K   | M     
```

For every item in LHS, determine the index of the item in RHS (-1 means not present in RHS)

```
LHS  | RHS
-----|------
A rindex=-1 | Bpre
B rindex=1  | B
C rindex=-1 | Bpost
D rindex=-1 | Hpre
E rindex=7  | H
F rindex=-1 | Hpost
G rindex=-1 | Epre
H rindex=4  | E
I rindex=-1 | Epost
J rindex=-1 | L
K rindex=-1 | M
```

Shift shared items in LHS with rindex >= 0 to ensure rindex is monotonically increasing
```
LHS  | RHS
-----|------
A rindex=-1 | Bpre
B rindex=1  | B
C rindex=-1 | Bpost
D rindex=-1 | Hpre
F rindex=-1 | H
G rindex=-1 | Hpost
H rindex=4  | Epre
E rindex=7  | E
I rindex=-1 | Epost
J rindex=-1 | L
K rindex=-1 | M
```

Merge LHS and RHS (iterator vars li and ri):
- If LHS is not empty and LHS[li].rindex == -1, take LHS[li] and increment li
- If LHS is not empty and LHS[li].rindex == ri, take RHS[ri] and increment li and ri
- If RHS is not empty, take RHS[ri] and increment ri