If F is a path from z to y then F$^-1$ is a path from y to z$.$

LEMMA $7\mathrm{.}10$ For each $[$F$]$ in $\backslash$Pi $(\backslash$Zeta $,$ z$),[$F$].[$F$^-1]=[$F$^-1].[$F$]=[$e$\_$z$]$