I hope I posted this in the right part of the forum...
Undefined terms: Fe's, Fo's, and the relation "belongs to."
Axiom 1. There exist exactly three distinct Fe's in this system.
Axiom 2. Any two distinct Fe's belong to exactly one Fo.
Axiom 3. Not all Fe's belong to the same Fo.
Axiom 4. Any two distinct Fo's contain at least one Fe that belongs to both.
Prove: There exists a set of two Fo's that contains all the Fe's of the system.