\cdbalgorithm{run}{}

Run an external program on the input expression, replacing it with the
standard output of the program. The expression is the first command
line argument of the program to be called.

Suppose we have a simple script {\tt test.sh} containing
\begin{screen}{0}
#!/bin/sh
echo "$1" | sed -e 's/A/B/g' -e '/;/q'
\end{screen}
This script replaces 'A' with 'B' in the first command line argument,
until it encounters a semi-colon.  We can then call this script by
using e.g.
\begin{screen}{1,2}
A B C A D E;
@run(%){"./test.sh"};
B B C B D E;
\end{screen}
The external program should make sure that it produces valid cadabra
input. 
