LBL"FRAC" is a short program, written, by Sherman Lowell, which
carries out exact arithmetic on rational fractions. It uses only
stack registers, no synthetics, and is therefore useful to call
as a subroutine. Either the Extended Function module or a HP-41CX
is needed because the GETKEY function is used.

At start the stack is loaded with two fractions: num1, denom1,
num2, denom2 in registers T,Z,Y,X. Execution of LBL"FRAC",
assigned to Sigma+ key in USER mode, prompts for binary
operations +, -, *, / assigned to those keys and unary operation
"RED", which reduces fraction to lowest terms, assigned to R key.
The result is available in ALPHA register (printed automatically,
if printer is attached) and as separate numerator and denominator
in Y- and X-registers. X<>Y