rcvr is a guard in the safeEnv indicating that the reference it guards is to be treated as eventual (i.e. a remote promise or far ref, but not a near ref) — immediate calls should not be made on it, but only eventual sends. This is not and cannot be enforced by the guard.
There has been discussion, but no resolution, of giving this concept a less cryptic name.
XXX Give formal documentation of guard object.