Retrieve the current definition of a key.
Create a derived environment where key
is bound to
value
; previous bindings of key
are shadowed.
Create a derived environment where key
is bound to
value
; previous bindings of key
are shadowed.
Clear the binding for the given key.
Clear the binding for the given key. Lookups for key
will be
negative in the returned environment.
Tells whether key
is defined in this environment.
Retrieve the current definition of a key, but only if it is defined.
Retrieve the current definition of a key if it is defined.
An environment without
key
. Lookups for other keys are forwarded tonext
.