I have a user defined function in a groovy script I keep in the Script Editor file list that I want to call from post function scripts also stored in the Script Editor list. My objective is to maintain one copy of the function that can be used across scripts (post functions). I have included the method's declaration to simplify the discussion.
def TransitionLinkedIssues(com.atlassian.jira.issue.Issue issue, String targetLinkTypeName, String targetCategoryName, String targetTransitionName) {
...
}