To elaborate an immutable reference just means the reference cannot be
reassigned but the pointed at object and all of its fields can be mutated so
the following happens:
var a = 100
a = 300 # Can reassign `a` (var int)
assert a == 300
let b = 100
#b = 300 # Cannot reassign `b` (let int)
let c = new int
c[] = 300 # We're assigning what `c` points to which is valid
assert c[] == 300
#c = new int # Cannot reassign `c` (let ref T)
var d = new int
d[] = 400
assert d[] == 400
d = new int # Can reassign `d` (var ref T)
Run
