title: Is it possible to use pointfree expressions in signature facts?
body: <p>Suppose we have the following alloy model:</p>
  
  <pre><code>sig A {}
  
  sig B  {
    R : A
    } 
  
  fact {
    R.~R in iden
    }
  
  run {}
  </code></pre>
  
  <p>On executing run, alloy finds an instance.  I thought I'd try to change the model's fact into a <em>signature fact</em>, as below:</p>
  
  <pre><code>sig A {}
  
  sig B  {
    R : A
      } {
      R.~R in iden
      }
  
  run {}
  </code></pre>
  
  <p>but when I do, alloy tells me:</p>
  
  <pre><code>A type error has occurred:
  ~ can be used only with a binary relation.
  Instead, its possible type(s) are:
  {this/A}
  </code></pre>