Godeps/_workspace/src/gopkg.in/mgo.v2/bson/bson.go
func (id ObjectId) Counter() int32 {
	b := id.byteSlice(9, 12)
	// Counter is stored as big-endian 3-byte value
	return int32(uint32(b[0])<<16 | uint32(b[1])<<8 | uint32(b[2]))
}
func (id ObjectId) Counter() int32 {
	b := id.byteSlice(9, 12)
	// Counter is stored as big-endian 3-byte value
	return int32(uint32(b[0])<<16 | uint32(b[1])<<8 | uint32(b[2]))
}
// Counter returns the incrementing value part of the id.
// It's a runtime error to call this method with an invalid id.