 No ontology chart node has more than two ontological antecedents.Where you find an arc on the ontology chart between a role name and a node, read that as an arc between the right hand side of the role name.So the arc from employee to works at is an arc between employment and works at

Mathematically, ontology charts are a graphical representation of semi-lattice structures; specifically they are Hasse diagrams of a single root and no cycles.