In some cases overlapping ranges of prefix regular expressions in a $in clause are not computed correctly:

> c.find( {x:{$in:[/^a/,/^ab/]}} ).explain()
{
"cursor" : "BtreeCursor x_1 multi",
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 1,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" :

{ "x" : [ [ "a", "ac" ], [ /^a/, /^a/ ], [ /^ab/, /^ab/ ] ] }

,
"server" : "Aaron-Staples-MacBook-Pro.local:27017"
}