You have to create a function,named `insertMissingLetters`, that takes in a `string` and outputs the same string processed in a particular way.
The function should insert **only after the first occurrence** of each character of the input string, all the **alphabet letters** that:
-**are NOT** in the original string  
-**come after** the letter of the string you are processing  
Each added letter should be in `uppercase`, the letters of the original string will always be in `lowercase`.
Example:  
`input`: "holly"  
`missing letters`: "a,b,c,d,e,f,g,i,j,k,m,n,p,q,r,s,t,u,v,w,x,z"  
`output`: "hIJKMNPQRSTUVWXZoPQRSTUVWXZlMNPQRSTUVWXZlyZ"  
You don't need to validate input, the input string will always contain a certain amount of lowercase letters  (min 1 / max 50).
from string import ascii_lowercase as a
import re
def insert_missing_letters(s):    
    t = ""
    c = set()
    for i in s:
        if i not in c:
            t = t + i + re.sub("|".join(s),"",a[a.index(i)+1:]).upper()
            c.add(i)
        else:
            t += i
    return t
from string import ascii_lowercase as a
import re
def insert_missing_letters(s):    
    t = ""
    c = set()
    for i in s:
        if i not in c:
            t = t + i + re.sub("|".join(s),"",a[a.index(i)+1:]).upper()
            c.add(i)
        else:
            t += i
    return t