Problem Statement  
You are given two Strings s1 and s2. Each string contains some letters and exactly one asterisk ('*'). You must replace each asterisk with a sequence of letters (possibly of zero length). The resulting two strings must be equal. Return the shortest possible resulting string. If it is impossible to make the strings equal, return "impossible" (quotes for clarity) instead.  
Definition  
 
Constraints  
  s1 will contain between 1 and 50 characters, inclusive.  
  s2 will contain between 1 and 50 characters, inclusive.  
  s1 and s2 will contain only uppercase letters ('A''Z') and asterisks ('*').  
  s1 and s2 will contain exactly one asterisk each.  
Examples  
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