# Functions
appendUsersFromFile appends the list of new users to the list of users from the XML file.
CountUsers returns the set of users as a map from username to the number of times that user appears in the list.
FromGitlabUser converts from gitlab.User to gilab_util.XmlUser by removing all the unnecessary user information.
FromGitlabUsers converts from gitlab.User slice to gilab_util.XmlUser slice by removing all the unnecessary user information.
ReadUsers reads the users from the XML file.
WriteUsers writes the users to the output file.