title: Not able to understand ANTLR parser rule
body: <p>I am new to ANTLR , and walking through existing grammar(got at Internet). See the given rule , i am not able to understand it for what is it all about?</p>
  
  <p>Specially  $model_expr inside Tree construct and initial (unary_expr -> unary_expr). Please help me understanding the same.</p>
  
  <pre><code>model_expr
  :    (unary_expr -&gt; unary_expr)
      (LEFT_BRACKET model_expr_element RIGHT_BRACKET
          -&gt; ^(MODEL_EXPR[$LEFT_BRACKET] $model_expr model_expr_element))?
  ;
  </code></pre>
  
  <p>Thanks</p>