In a toxicological study with six treatments T$1,...,$T$6($corresponding to an increasing dose$),$ in pairwise comparisons the following differences are significant: T$1$ vs$.$ T$4,$ T$1$ vs$.$ T$5,$ T$1$ vs$.$ T$6,$ T$2$ vs$.$ T$5,$ T$2$ vs$.$ T$6,$ and T$3$ vs$.$ T$6.$ Perform the insert$-$and$-$absorb algorithm by hand to derive the corresponding letter display.