If I understand correctly, it would be best to break down the input into smaller inputs.

• If the input has only one of each of 4 subsequent letters (e.g. "A", "D", "E" and "F"), then they must form a tupple if there's a solution. Find the solutions without these letters present, and add this tupple to each of them.

• If the input has none of a letter (e.g. "G"), then you can find the solutions for everything up to that letter and the solutions for everything above that letter separately, and find their product.

Update: Oops, the first one isn't quite right. Consider when there is 5 subsequent letters with a count of 1.