@EFFect <exit> [= (Add|Remove|Toggle) (<group>) [:<target>]], ...
   or
@EFFect <exit> = Delete <Effect Number>

Causes an <exit> to Add, Remove or Toggle a group on the player
using the exit by default, or on a specified <target>.

Several effects can be associated with an exit.
@EFFECT <exit> by itself will display an exit's existing effects.

The ":" operator can be used in locks to verify that <target> is
a member of <group>.

The NOTIFY effect is discussed separately see NOTIFY

see also: @add, @lock, @remove, groups, membership, operators,
    targets, toggle, notify