title: Context-Free Grammar suggestions for unary subtraction?
body: <p><code>   L = {1<sup>i</sup> - 1<sup>j</sup> = 1<sup>i-j</sup>: i-j >= 0, i,j>=0} </code></p>
  
  <p>I'm confused about how to construct a grammar that tracks subtraction of a string element. I have no clue on how to get started on this and attempted to work with an equivaent construction of the form</p>
  
  <p><code>   L = {1<sup>i</sup> = 1<sup>i-j</sup> + 1<sup>j</sup>} </code></p>
  
  <p>Any hints or suggrstions are appreciated. </p>