## Golfscript, 8 chars


This defines a function called `A`

 {$\$=}:A

Test cases

 ;
 'boat' 'boat' A
 'toab' 'boat' A
 'oabt' 'toab' A
 'a' 'aa' A
 'zzz' 'zzzzzzzz' A
 'zyyyzzzz' 'yyzzzzzy' A
 'sleepy' 'pyels' A
 'p' 'p' A