Operators &=

Assigns x to the value of x & b. It is equivalent to x = x & b. This is often used for disabling bitflags in a bitfield.

#define MY_BITFLAG (1<<0)

var/my_field = MY_BITFLAG // starts enabled
my_field &= ~MY_BITFLAG // now disabled

If both x and y are /lists, items in x that are not present in y will be removed.

var/list/a = list("foo")
var/list/b = list("bar")

a &= b

world.log << json_encode(a) // empty list!