Problem Statement

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If the program uses UTF-8 or UTF-16,   it must work on any valid Unicode code point, whether in the   Basic Multilingual Plane   or above it. The program must reference logical characters (code points),   not 8-bit code units for UTF-8 or 16-bit code units for UTF-16. Programs for other encodings (such as 8-bit ASCII, or EUC-JP) are not required to handle all Unicode characters.

Let's start with the solution:

: substrings(s, n, m)
   s sub(n, m) println
   s right(s size n - 1 +) println
   s left(s size 1 - ) println
   s sub(s indexOf('d'), m) println
   s sub(s indexOfAll("de"), m) println ;