What could $id_{D(R)}$ and $id_{W(R)}$ mean in context of relation and reverse relation

Question

Show that for a relation $R$ with a reverse relation $R^{-1}$ and $id_M:=\{(x,x)\in M\times M|x\in M\}$ following holds:

a) $id_{W(R)}\subset R\circ R^{-1}$

b) $id_{D(R)}\subset R\circ R^{-1}$

I don't know how those identities are defined and can't find them anywhere.

That makes it impossible for me to solve this.

Answer 1

$id_M$ is the identity relation on the set $M$.

It seems that $id_{D(R)}$ is the identity relation on the domain of $R$.

If so, from the context we have to assume that $id_{W(R)}$ is the identity relation on the codomain.