There is a population of people (finite or infinite). Any group of people (finite or infinite) in this population has someone assigned to be in charge of them. So someone is in charge of the whole population, someone is in charge the set of no people, each person has someone in charge of them (which may or may not be themselves) etc.
If the person in charge of a group of people belongs to the group, we call them the leader of the group. If the person in charge of a group of people does not belong to the group, we call them the ruler of that group.
Show that someone is both a ruler and a leader.