If 
is true precisely unless 
, then 
implies not-
and not-
implies 
. J. H. Conway has suggested the term "unlesss"
for this state of affairs, by analogy with iff.
Precisely Unless
See also
At Least One, Exactly One, Iff, UnlessExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Precisely Unless." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PreciselyUnless.html