Well, there is a list of mutating operators: +=, -=, *=, etc.
Some similar operators (using =) are not: <=, >=
On the other hand, I think that this combination is not used jet: +>, <+
a <+ factorial(5) # a = [1,2,3,120]
The less sign (<) gives some notion of direction and the + sign some notion of addition.
Well, this is just an idea and probably will stay as that because the reasons already presented.