docs: function visitRequireArgument(ast, cb) {
  	types.visit(ast, {
  		visitCallExpression: function(path) {
  			if (this.isRequireExpression(path.node)) {
  				var arg = path.getValueProperty("arguments")[0];
  
  				if (n.Literal.check(arg)) {
  					cb(arg);
  				}
  			}
  
  			this.traverse(path);
  		},
  
  		isRequireExpression(node) {
  			return n.Identifier.check(node.callee) && node.callee.name === "require";
  		}
  	});
  }