meaning of '+='

enum nephews { hughie, louis, dewey }

open util/ordering[nephews] as nephewsOrd

abstract sig nephews {}

one sig hughie extends nephews {}
one sig louis extends nephews {}
one sig dewey extends nephews {}

fact {
  nephewsOrd/first = hughie
  nephewsOrd/next = hughie -> louis + louis -> dewey
}

>>> [myList[each] for each in [1, 0, 4, 2, 3]]
[1, 2, 6, 7, 3]

>>> [myList[each] for each in [1, 0, 3, 4, 2]]
[1, 2, 3, 6, 7]

myList = [2, 1, 7, 3, 6]

def get_rankings(list):
    return [sorted(list).index(each) for each in list]

print get_rankings(myList)
#[1, 0, 4, 2, 3]