, it follows that we can use ${\Delta}'_b$ as a desired diagram ${\Delta}_c$ for $c$.  (0,0) circle (0); (0,-0) -(0,-0); (0,-0) -(0,-0.0); (-.0,0) -(.0,0); (0,-0) \[fill = black\] circle (.00); (0,-0) \[fill = black\] circle (.00); at (0,-0.0) [Fig. 0.0]{}; at (0,-.0) [$\Delta_b$]{}; at (-.0,-0.0) [$f$]{}; at (0,.0) [$