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.