Call to undefined function VariableNode::further() in pharborist

PHP Fatal error: Call to undefined method Pharborist\Variables\VariableNode::furthest() in /var/www/html/drupal-8/modules/drupalmoduleupgrader/src/Plugin/DMU/Rewriter/Generic.php on line 142
Drush command terminated abnormally due to an unrecoverable error. [error]
Error: Call to undefined method Pharborist\Variables\VariableNode::furthest() in
/var/www/html/drupal-8/modules/drupalmoduleupgrader/src/Plugin/DMU/Rewriter/Generic.php, line 142

The above is the error message when trying to upgrade pants module with dmu-upgrade command. After reading the code for VariableNode.php here : https://github.com/grom358/pharborist/tree/master/src/Variables , I found out that the function furthest() has not been defined in VariableNode.php

Th function is being called in drupalmoduleupgrader/src/Plugin/DMU/Rewriter/Generic.php, line 142 and it flags an error as it supposed to. Either the code is incomplete in VariableNode.php or Generic.php is incorrect.

Urgent response required.