So if we enter the for loop, we know there is a difference between the two sequences.
So we just need to find an index position i where (isnt seq1.i seq2.i) is true. If indexing off the end of a string causes an error, we need to put in code to prevent that. If indexing off the end of a string returns anything that can't be an element of the string, we can compare each index i until we find a difference, knowing that we'll eventually find one.
If the string X = the string Y, print 0 (to indicate there are no differences / MUMPS uses 1-based indexing of strings)
Otherwise, go through each character of the strings and at the point where they differ, print that index and quit looping.
MUMPS does not error out on attempting to extract a character beyond the string's length. So in that event, 1 string's extract will return the empty string, which will differ from the other string's extract and will cause you to print the index and quit. Not generating such an error, of course, cuts down on the code needed.