def sum(ptr, n, a)
  n ? sum(ptr, n-1, a + ptr[n-1]) : a

ifz n goto L
r1 = r1 - 1
r4 = r1 - 1   # without CSE
r3 = ptr[r4]
r2 = r2 + r3
goto 0        # loopification
L:
r0 = r2
return