title: SML - if's condition with some "or"
body: <p>I want to make a if condition like this:</p>
  
  <pre><code>if
  
   ((head(c) = 1) or (head(c) = ~1) or (head(c) = ~5) or (head(c) = ~17) or (head(c) = 0))
  count +1
  else..
  </code></pre>
  
  <p>the function head return 'a;</p>
  
  <p>It gives me the next error: <code>operator is not a function [tycon dismatch]</code>
  operator: bool
  in expression  </p>
  
  <p>What is the problem? thank you.</p>