The receiver for the GR domain. This receiver is a mailbox with capacity one,
and any token put in the receiver overwrites any token previously present in
the receiver. As a consequence, the hasRoom() method always returns true. The
get() method will consume the token if there exists one. After the
consumption, the hasToken() method will return false, until a token is put
into this receiver.
Put a token into this receiver. If the argument is null,
then this receiver will not contain a token after this method
returns. If the receiver already has a token, then the new token
will override the old token, and the old
token will be lost.