## sort_sum

Sort terms in a sum.

Sort terms in a sum, taking into account any `SortOrder`

properties, or else sorting lexographically. ex:=E+D+A+C+B;

\(\displaystyle{}E+D+A+C+B\)

sort_sum(_);

\(\displaystyle{}A+B+C+D+E\)

This is often useful in case sums appear as exponents; in this case it
is necessary to first sort the sums before terms can be collected, as
the following example shows.

ex:=a**(-1+d) - a**(d-1);

\(\displaystyle{}a^{\left(-1+d\right)}-a^{\left(d-1\right)}\)

sort_sum(_);

\(\displaystyle{}0\)