GRPN is a graphical reverse polish notation (RPN) calculator.
GRPN is written in C and uses the GIMP Toolkit (GTK) on top of X11.  GRPN
was developed under Linux but has also been tested under SunOS and Solaris.
It should work with almost any UNIX, though. GRPN works with real numbers
and complex numbers.  Numbers can be displayed in 4 different radix modes,
and complex numbers can be displayed in either Cartesian or polar form.

GRPN uses a stack and reverse polish notation to evaluate expressions.  The
stack in GRPN is infinite and limited only by available memory.