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   Date: Tue, 20 Apr 2004 19:57:27 -0400 (EDT)
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   We want to find out the first position $n at which strings $x and $y
   differ.  If $x eq $y, then $n is undef.  If $x ne $y but $x is a
   prefix (aka initial segment) of $y, then $n should be equal to length
   $x.  The operation must preserve both $x and $y (i.e. destructive
   operations are disallowed).

   By way of baseline, here's an utterly uninspired solution:

   $n=0;while(($c=substr($x,$n,1)||undef $n) && $c eq substr($y,$n++,1)){;}

It's worse than uninspired, actually.  It's wrong.

kj

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