The left unitor of $\mathsf{Rel}$ is the natural isomorphism
\[ \lambda ^{\mathsf{Rel}}_{A} \colon \mathbb {1}_{\mathsf{Rel}}\times A \mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}A \]
at $A$ is defined by declaring
\[ \webleft (\star ,a\webright ) \sim _{\lambda ^{\mathsf{Rel}}_{A}} b \]
iff $a=b$.
