Illustration of the cases in Theorem 1. The graph in the figure represents $G_{\mathrm{basic}}\left(\mathrm{s}{\mathrm{p}}^k\left(\mathcal{S}\right)\right)$, where k = 3 and $\mathcal{S}=\left\{\mathrm{CTTGG},\mathrm{CTTGACTT},\mathrm{TACTT},\mathrm{TGAC}\right\}$. The segments are marked by dashed lines with their starts marked with a dot and their ends marked with a diamond. The unitig w = {ACT, CTT, TTG} is unsafe because for each of the segment, one of the cases in Theorem 1 is true. For segment colored in green, case 1 holds; for red, case 2; for blue, case 3; and for orange, case 4.