title: what does "Configuration is valid for a viable prefix " means?
body: <p>I am learning LR(1) parsing from <strong>THE DRAGON BOOK</strong>. I am really confused with this sentence.</p>
  
  <p>Formally we say that a configuration <strong>[A –> u•v, a]</strong> is valid for a viable prefix <strong>α</strong> if 
  there is a rightmost derivation <strong>S =>* βAw =>* βuvw</strong> where <strong>α = βu</strong> and either <strong>a</strong> is the first 
  symbol of <strong>w</strong> or <strong>w</strong> is <strong>∂</strong> and <strong>a</strong> is <strong>$</strong>.  </p>
  
  <p>I am not able to figure out how validation of a configuration for a viable prefix is <strong><code>useful</code></strong> .
  What does it speak <strong><code>intutively</code></strong> and why it <strong><code>matters</code></strong> ???</p>
  
  <p>Please help .... </p>