Solution
= Solution
A <metric> dissimilarity must satisfy, for all $x,y,z$, the four properties
$$
\boxed{d(x,y)\ge0,\qquad d(x,y)=0\ \Longleftrightarrow\ x=y,\qquad
d(x,y)=d(y,x),\qquad d(x,z)\le d(x,y)+d(y,z).}
$$
They are nonnegativity, identity of indiscernibles, symmetry and the <triangle inequality>. The following two transformations of the <simple matching coefficient> behave differently at the identity.