def array_minus(minuend, subtrahend)
      return minuend - subtrahend unless rbx?
      set = Set.new(minuend) - subtrahend
      minuend.select {|e| set.include?(e)}
    end
def array_minus(minuend, subtrahend)
      return minuend - subtrahend unless rbx?
      set = Set.new(minuend) - subtrahend
      minuend.select {|e| set.include?(e)}
    end
Returns a sub-array of `minuend` containing only elements that are also in
 `subtrahend`. Ensures that the return value has the same order as
 `minuend`, even on Rubinius where that's not guaranteed by `Array#-`.

 @param minuend [Array]
 @param subtrahend [Array]
 @return [Array]