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Exercise 2.9
Letter case conversion **

In many web documents it is desirable to control the letter case of selected words. This allows us to present documents with consistent appearances. Therefore it is helpful to be able to capitalize a string, to transform a string to consist of upper case letters only, and to lower case letters only. Be sure to leave non-alphabetic characters untouched. Also, be sure to handle the Danish characters 'æ', 'ø', and 'å' (ASCII 230, 248, and 229 respectively). In addition, let us emphasize that we want functions that do not mutate the input string by any means. (It means that you are not allowed to modify the strings passed as input to your functions).

Write functions capitalize-a-string, upcase-a-string, downcase-a-string for these purposes.

As examples of their use, please study the following:

    (capitalize-a-string "monkey") => "Monkey"

    (upcase-a-string "monkey") => "MONKEY"

    (downcase-a-string "MONkey") => "monkey"

Hint: I suggest that you program the necessary functions yourself. Convert the string to a list of ASCII codes, do the necessary transformations on this list, and convert the list of modified ASCII codes back to a string. The Scheme functions list->string and string->list are useful.

Hint: If you want to make life a little easier (and learn less from this exercise...) you can use the Scheme functions char-upcase and char-downcase, which work on characters. But these functions do maybe not work on the Danish letters, so you you probably need some patches.