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sort a list
SortBy(V:LIST, F:FUNCTION):NULL
where V is a variable containing a list and F is a booleanvalued
comparison function of two arguments (e.g. representing less than).

This function sorts the elements of the list in V with respect
to the comparisons made by F; it overwrites V and returns NULL.
The comparison function F takes two arguments and returns TRUE if the
first argument is less than the second, otherwise it returns FALSE.
The sorted list is in increasing order.
Note that if both F(A, B) and F(B, A) return TRUE, then A and B are
viewed as being equal.
Define ByLength(S, T)  define the sorting function
Return Len(S) > Len(T);
EndDefine;
M := ["dog","mouse","cat"];
SortBy(M, Function("ByLength"));
M;
["mouse", "dog", "cat"]

