title: CFG and closure properties
body: <p>I am solving one problem and I urgently need a hint to solve one problem:</p>
  
  <blockquote>
    <p>Use closure under union to show that the following language is context-free:</p>
    
    <p>{a<sup>m</sup> b<sup>n</sup> c<sup>p</sup> d<sup>q</sup> : n=q or m &lt;= p or m+n=p+q }</p>
  </blockquote>