title: Valid expressions to the quote function
body: <p>According to the manual of <code>quote(expr)</code>:</p>
  
  <blockquote>
    <p>expr: any syntactically valid R expression</p>
  </blockquote>
  
  <p>While <code>quote(x==y)</code> returns a call <code>x==y</code> successfully, <code>quote(x=y)</code> fails:</p>
  
  <pre><code>Error in quote(x = y) : supplied argument name 'x' does not match 'expr'
  </code></pre>
  
  <p>Both <code>x=y</code> and <code>x==y</code> are syntactically valid R expressions, aren't they? Why <code>quote()</code> fails on <code>x=y</code>?</p>