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depth:                    X x Y                             required m
+emory:
  1:                      4 x 4                      ==      16 _b
...
 30:    1152921504606847000 x 1152921504606847000    ==   1 2^120b
 31:    4611686018427387900 x 4611686018427387900    ==  16 2^120b
 32:   18446744073709552000 x 18446744073709552000   == 256 2^120b

if( X == Y ) {
    return depth;
...
    depth--;
    recurse;
}

#! perl -slw
use strict;
...
    }
    print '';
}