The horizontal composition functor of $\mathsf{Rel}^{\mathsf{dbl}}$ is the functor
of $\mathsf{Rel}^{\mathsf{dbl}}$ is the functor where
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Action on Objects. For each composable pair $\smash {A\mathbin {\overset {R}{\mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}}}B\mathbin {\overset {S}{\mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}}}C}$ of horizontal morphisms of $\mathsf{Rel}^{\mathsf{dbl}}$, we have
\[ S\mathbin {\odot }R\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}S\mathbin {\diamond }R, \]where $S\mathbin {\diamond }R$ is the composition of $R$ and $S$ of Chapter 9: Constructions With Relations,
.
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Action on Morphisms. For each horizontally composable pair
of $2$-morphisms of $\mathsf{Rel}^{\mathsf{dbl}}$, i.e. for each pair
of inclusions of relations, the horizontal composition
of $\alpha $ and $\beta $ is the inclusion of relations1
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1This is justified by noting that, given $\webleft (a,c\webright )\in A\times C$, the statement
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We have $a\sim _{\webleft (U\mathbin {\diamond }T\webright )\circ \webleft (f\times h\webright )}c$, i.e. $f\webleft (a\webright )\sim _{U\mathbin {\diamond }T}h\webleft (c\webright )$, i.e. there exists some $y\in Y$ such that:
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We have $f\webleft (a\webright )\sim _{T}y$.
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We have $y\sim _{U}h\webleft (c\webright )$.
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We have $a\sim _{S\mathbin {\diamond }R}c$, i.e. there exists some $b\in B$ such that:
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We have $a\sim _{R}b$.
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We have $b\sim _{S}c$.
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If $a\sim _{R}b$, then $f\webleft (a\webright )\sim _{T}g\webleft (b\webright )$, as $T\circ \webleft (f\times g\webright )\subset R$;
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If $b\sim _{S}c$, then $g\webleft (b\webright )\sim _{U}h\webleft (c\webright )$, as $U\circ \webleft (g\times h\webright )\subset S$.
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