That was neat. Here's an implementation in Python:
# Python implementation of Conway's prime formula for FRACTRAN
# The output is the sequence A034785.
from fractions import Fraction as F
terms = [F(17, 91), F(78, 85), F(19, 51), F(23, 38), F(29, 33),
F(77, 29), F(95, 23), F(77, 19), F(1, 17), F(11, 13),
F(13, 11), F(15, 14), F(15, 2), F(55, 1)]
n = 2
print n
while 1:
for i, f in enumerate(terms):
fn = f * n
if fn.denominator == 1:
n = fn.numerator
print n
break
else:
break
