Definition.
Write
τ
X
,
Y
[
U
|
Z
;
V
|
W
]
for
∑
Z
,
W
ℙ
[
Z
=
z
]
ℙ
[
W
=
w
]
τ
X
,
Y
[
U
|
Z
=
z
;
V
|
W
=
w
]