\cdbalgorithm{factorise}{}

Given a list of symbols, this algorithm collects terms in a sum that
only differ by pre-factors consisting of these given symbols. As an
example,
\begin{screen}{1,2}
a b + a c + a d:
@factorise!(%){b,c};
(b + c) a + a d;
\end{screen}
This is like the {\tt antibracket} statement of FORM. A better name is
probably {\tt factor\_in}.

The algorithm of course also works with indexed objects, as in
\begin{screen}{1,2}
A_{m} B_{m} + C_{m} A_{m};
@factorise!(%){B_{n},C_{n}};
(B_{m} + C_{m}) A_{m};
\end{screen}
~
