docs: function parseExtend (ast) {
      for (let i = 0, max = ast.body.length; i < max; i++) {
          let node = ast.body[i];
  
          // Search for extend('Parent', 'Child');
          if (isExpressionStatement(node) && isCallExpression(node.expression) && node.expression.callee.name === 'extend') {
              node = node.expression;
  
              let parent = node.arguments[0].value;
              let child = node.arguments[1].value;
  
              createHierarchyNode(parent);
              createHierarchyNode(child);
  
              // Link child to parent.
              hierarchy[parent].children.push(child);
  
              // Link parent to child.
              hierarchy[child].parent = parent;
  
              return true;
          }
      }
  
      return false;
  }