If we eliminate char from HtDP/I, we need to add re-think the following
functions: 

integer->char -- 1string version 

char->integer -- 1string version

string->list -- explode 

list->string -- implode 

char-numeric?  -- in a sense string->number is enough
 (number? (string->number s))

char-alphabetic?  -- 
 (andmap (lambda (c) 
           (or (string<=? "A" x "Z") (string<=? "a" x "z")))
         (string->list s))

char-whitespace?  -- (andmap char-whitespace? s)

char-upper-case?  -- (string<=? "A" x "Z")

char-lower-case?  -- (string<=? "a" x "z")

char-upcase   string-upcase

char-downcase string-downcase

make-string : Nat Char -> String 
 Nat String1 -> String 

string      : Char ... -> String 
 delete, string-append is enough 

string-ref  : String Nat -> Char
 ith

NOTE: 
substring consumes 2 or 3 arguments