Maybe something more along the lines of this will help:
a = [:bears, :elephants]
b = a
a = Enum.reject(a, fn animal -> animal == :bears end)
IO.inspect(a)
IO.inspect(b)
So we make a point to some value, and then make b point to the same thing as a. Then we call Enum.reject the result of which a gets rebound to. The reject call creates a new list based on what a was originally bound to: it cannot modify what a was originally bound to since that original list is immutable. b is still bound to the original list that a was originally bound to. Since that original list is immutable, we can make assumptions about b without concern with what happens to a later on. a itself may be rebound, but that original list of [:bears, :elephants] is not changed as a result.
