Please correct my statement.Assuming $L\in NP$, and algorithm A can determine L in poly-time in a nondeterministic machine, we have algorithm $A'$ and the complement of $L$ -- $L'$. $x$ is the input of $A'$A'(x){   if(A(x) is true)      return false   else      return true}In this code, it seems like $A'$ can also run in a nondeterministic machine in poly-time. Can I just say $co-NP=NP$??Or my flaw is that the input $x\in L'$ but $x\notin L$?Could you please give me a specific example?? 
Why NP is not closed under complement?
complexity theory;computability;np complete;decision problem