The char datatype can be treated as its unicode equivalent without typecasting or converting. So it's is evaluating the chars based on their unicode equivalents. If you're not familiar with Unicode you definetly need to research it.

whenever you index a string it returns the char, and a char can't be concatonated to another char, but it can be concatonated to a string so the second index doesn't need to be converted to a string.

I can tell what retval means, I can understand why you would name your string that. However, I have an intense dislike for it.