Previously, we computed
by computing
. Could we have computed
instead? No. Since
applied to
is
![](Images/indent.png)
the
which applies to
would be
![](Images/indent.png)
which is undefined. The order of the shift and inverse shift operators is therefore
important.
Definition
A sequence
of shift and inverse shift operators
,
,
, and
is a
proper sequence
if the composition
is defined.
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