title: Use pumping lemma to prove grammar is not context free?
body: <p>I'm trying to prove that <code>L={y#x|(y is a substring of x)  ∧x,y∈{a,b}^* }</code> is not context free using the pumping lemma, but I can't seem to do that. If  </p>
  
  <pre><code>|vy|≠ε ,|vxy|≤k , uv^n xy^n z∈L ,∀n≥0
  </code></pre>
  
  <p>Then either <code>vxy</code> has both <code>a</code> and <code>b</code>, or only <code>b</code> or only <code>a</code>.</p>
  
  <p>How can I pump it in order to show that?</p>