// So that an element with the class 'a_parent' opens 
// 'a_child', and 'b_parent' opens 'b_child' etc.

function listen(parent_id, child_id) {

  // Grab the base element to listen for
  var parent = document.getElementById(parent_id);

  // Listen on that element for the click to open the child.
  parent.onclick = function (child_id) {
     
     // Grab and check the child's style.
     var child = document.getElementById(child_id);
     
     // You could check the .style.height, or .style.visibility
     if (child.style.display === 'none') {
          
          // Show the child element
          // You could use inline, inherit, or auto (CSS properties)
          child.style.display = 'block';
          
     } else {
          
          // Hide the child
          child.style.display = 'none';
          
     }

   }
}