title: Scheme - Check if element of list is a single letter
body: <p>I have a list <code>`(+ 1 2 3 a b c)</code></p>
  
  <p>I want to check if an element is a letter.</p>
  
  <p><code>(symbol? (list-ref mylist idx))</code> will return <code>#t</code> for both <code>a</code> and <code>+</code>, which I don't want.</p>
  
  <p>Perhaps I could compare the element to <code>[a-zA-z]</code>, or check if it's a symbol that's not <code>[+-*/]</code>, but that seems tedious.</p>
  
  <p>How is this done?</p>
  
  <p><strong>Update:</strong></p>
  
  <p>Actually, I could just write my own function.</p>
  
  <pre><code>(define is-letter
    (lambda (c)
      (if (and (not (number? c))
           (not (eq? c `+)) (not (eq? c `-)) (not (eq? c `*)) (not (eq? c '/)))
        #t
        #f
      )
    )
  )
  </code></pre>