(7,18): Error: index out of range
  |
7 |              else s[IndexOf(s, r.value)+1] == x
  |                    ^^^^^^^^^^^^^^^^^^^^^^^
(7,20): Error: function precondition could not be proved
  |
7 |              else s[IndexOf(s, r.value)+1] == x
  |                     ^^^^^^^^^^^^^^^^^^^

DafnyStandardLibraries.dfy(6627,13): Related location
     |
6627 |     requires v in xs
     |              ^^^^^^^

(19,4): Error: a postcondition could not be proved on this return path
   |
19 |     return Some(s[v-1]);
   |     ^^^^^^

(7,18): Related location: this is the postcondition that could not be proved
  |
7 |              else s[IndexOf(s, r.value)+1] == x
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

(19,16): Error: index out of range    
   |
19 |     return Some(s[v-1]);