This doesn’t exist yet, but it’s because each variable can only have one value at a time:
x := (y | 2) # y doesn't exist yet, we'll keep searching
y := (7 | 8) # Now x has a value
(x, y)
(7, 7) # We start with the first value of x and the first value of y, x = y and y = 7, so x = 7 too
(8, 8) # We stick with the first value of x and move to the second value of y, x = y still and now y = 8, so x = 8 as well (since y is always one value)
(2, 7) # The second value of x, and the first value of y
(2, 8) # The second value of x and the second value of y