for all pairs of states 
and 
in the chart.
Then
The inner probability 
is not used.
Rationale.
Relative to 
, 
is missing the probability of
expanding Y, which is filled in from 
.
The probability of the surrounding of Y ( 
) is the probability of
the surrounding of X ( ), plus the choice of the rule of production
for X and the expansion of the partial LHS 
, which are
together given by 
.
Note that the computation makes use of the inner probabilities computed in
the forward pass.
The particular way in which and were defined turns out
to be convenient here, as no reference to the
production probabilities themselves needs to be made in the computation.
As in the forward pass, simple reverse completion would not terminate in the presence of cyclic unit productions. A version that collapses all such chains of productions is given below.