type JumpBuffer {.importc: "jmp_buf", header: "<setjmp.h>".} = object

proc setJump(env: JumpBuffer): cint
   {.importc: "setjmp", header: "<setjmp.h>".}

proc longJump(env: JumpBuffer, value: cint)
   {.importc: "longjmp", header: "<setjmp.h>".}

var
   buf: JumpBuffer
   a: cint = 0
   b: cint = 1

echo setJump(buf)
b = a+b
a = b-a
if a < 500: longJump(buf, a)
echo "done"