\cdbproperty{Diagonal}{}

Indicates that the object to which the property is attached only has
non-zero components on the diagonal, i.e.~when all indices take the
same value. 
\begin{screen}{1,2}
\delta_{m n}::Diagonal.
\delta_{1 2} \delta_{1 2} - \delta_{1 1} \delta_{2 2};
@canonicalise!(%);
- \delta_{1 1}\delta_{2 2};
\end{screen}
~

\cdbseeprop{Coordinate}
