title: Lua coroutine.yield(-1)'s meaning?
body: <p>What does <code>coroutine.yield(-1)</code> mean? I don't understand <code>-1</code> here.</p>
  
  <p>code piece and out put is:</p>
  
  <pre><code>&gt; function odd(x)
  &gt;&gt;   print('A: odd', x)
  &gt;&gt;   coroutine.yield(x)
  &gt;&gt;   print('B: odd', x)
  &gt;&gt; end
  &gt;
  &gt; function even(x)
  &gt;&gt;   print('C: even', x)
  &gt;&gt;   if x==2 then return x end
  &gt;&gt;   print('D: even ', x)
  &gt;&gt; end
  &gt;
  &gt; co = coroutine.create(
  &gt;&gt;   function (x)
  &gt;&gt;     for i=1,x do
  &gt;&gt;       if i==3 then coroutine.yield(-1) end
  &gt;&gt;       if i % 2 == 0 then even(i) else odd(i) end
  &gt;&gt;     end
  &gt;&gt;   end)
  &gt;
  &gt; count = 1
  &gt; while coroutine.status(co) ~= 'dead' do
  &gt;&gt;   print('----', count) ; count = count+1
  &gt;&gt;   errorfree, value = coroutine.resume(co, 5)
  &gt;&gt;   print('E: errorfree, value, status', errorfree, value, coroutine.status(co))
  &gt;&gt; end
  ----    1
  A: odd  1
  E: errorfree, value, status     true    1       suspended
  ----    2
  B: odd  1
  C: even 2
  E: errorfree, value, status     true    -1      suspended
  ----    3
  A: odd  3
  E: errorfree, value, status     true    3       suspended
  ----    4
  B: odd  3
  C: even 4
  D: even         4
  A: odd  5
  E: errorfree, value, status     true    5       suspended
  ----    5
  B: odd  5
  E: errorfree, value, status     true    nil     dead
  &gt;
  </code></pre>