The tt(SYSTEM) macro takes one argument: a command to execute. The command is
run via the standard C function tt(system). The presence of this macro in the
Yodl language introduces the danger of em(live data); imagine someone sending
you a document with

verb(SYSTEM(rm *))

in it. To avoid such malevolent side effects, the tt(yodl) program has a flag
tt(-l) to define the `live data policy'. By default, tt(-l0) is implied which
suppresses the tt(SYSTEM) macro and the related tt(PIPETHROUGH) macro. See also
section ref(using).

Despite the potential danger, tt(SYSTEM) can be useful in many ways. E.g., you
might want to log when someone processes your document, as in:

verb(SYSTEM(echo Document processed! | mail myself@my.host))

