% Things dealing with arrows in a category. Or functions, basically.
%
+\input{mjo-common}
+
% The identity operator/arrow on its argument.
\newcommand*{\identity}[1]{ \operatorname{id}_{{#1}} }
% The inverse of an arrow, function, or whatever.
\newcommand*{\inverse}[1]{ #1^{-1} }
+
+% The preimage of the second argument (a set) under the first (a function).
+\newcommand*{\preimage}[2]{ #1^{-1}\of{#2} }