I have installed the module Drush Help to see the module's Drush commands in the module help page. But when I go to see the help page of a module that use in the drush commands drush constants like DRUSH_BOOTSTRAP_DRUPAL_FULL I got a Notice because this constant is only defined in drush, and not for Drupal.
Notice: Use of undefined constant DRUSH_BOOTSTRAP_DRUPAL_FULL - assumed 'DRUSH_BOOTSTRAP_DRUPAL_FULL' in node_revision_delete_drush_command()
So the problem is that Drupal can't reach the file includes/bootstrap.inc where the constant is defined. I can include the file but the problem is that I need to know where Drush is installed.
How to find where Drush is installed to include a file in my module?