function foo (a)
    print("foo", a)
    return coroutine.yield(2*a)
end

co = coroutine.create(function (a,b)
   print("co-body1", a, b)
   local r = foo(a+1)
   print("co-body2", r)
   local r, s = coroutine.yield(a+b, a-b)
   print("co-body3", r, s)
   return b, "end"
end)

print("1----")
print("main", coroutine.resume(co, 1, 10))
print("2----")
print("main", coroutine.resume(co, "r"))
print("3----")
print("main", coroutine.resume(co, "x", "y"))
print("4----")
print("main", coroutine.resume(co, "x", "y"))

1------
co-body1    1   10
foo 2
main    true    4
2------
co-body2    r
main    true    11  -9
3------
co-body3    x   y
main    true    10  end
4------
main    false   cannot resume dead coroutine

print("main", coroutine.resume(co, 1, 10))

print("main", coroutine.resume(co, "r"))

print("main", coroutine.resume(co, "x", "y"))

print("main", coroutine.resume(co, "x", "y"))