-must be no three superclasses $X$, $Y$ and~$Z$ of $C$ such that $Z$ is the
-link superclass of both $X$ and $Y$. As a consequence of this rule, the
-superclasses of $C$ can be partitioned into linear \emph{chains}, such that
-superclasses $A$ and $B$ are in the same chain if and only if one can trace a
-path from $A$ to $B$ by following superclass links, or \emph{vice versa}.
+must be no three distinct superclasses $X$, $Y$ and~$Z$ of $C$ such that $Z$
+is the link superclass of both $X$ and $Y$. As a consequence of this rule,
+the superclasses of $C$ can be partitioned into linear \emph{chains}, such
+that superclasses $A$ and $B$ are in the same chain if and only if one can
+trace a path from $A$ to $B$ by following superclass links, or \emph{vice
+versa}.