Definition. Track [math-GM4F]

For assemblies $X$ and $Y$, we say that an element $t \in \mathcal{A}$ tracks a function $f : |X| \to |Y|$ if for all $x \in |X|$ and $a \in \mathcal{A}$, if $a \Vdash_X x$, then $t\ a$ is defined and $t\ a \Vdash_Y f(x)$.