In C$++.$Write a complete program using nested for loops that prints out all the four letter strings that use some combination of distinct letters in the range $\{\text{'}$a$\text{'},..,$ last$\}$ to the console, one per line, where last is a user supplied letter. Only strings in which the letters are in ascending order will be printed.

$*$REQUIREMENT$*$

only use $15$ lines of code.