Function: extern
Section: programming/specific
C-Name: gpextern
Prototype: s
Help: extern(str): execute shell command str, and feeds the result to GP (as
 if loading from file).
Doc: the string \var{str} is the name of an external command (i.e.~one you
 would type from your UNIX shell prompt). This command is immediately run and
 its output fed into \kbd{gp}, just as if read from a file.
