Question: Shift letters by 3 using Stringtools

Simple question: How do I shift a letter (or a set of letters) over 3 or 7 or whatever places in the alphabet using with(Stringtools); Without converting the letters to numbers? Is there a shift function? or do i have to create a whole new alphabet to charactermap to for each shift I want to do. It seems very easy but I can't quite figure it out. Any help would be appreciated.
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