| \node[] (i0j1)  at (-1, 1) {A}; | |
| \node[] (i1j1) at (1, 1) {C}; | |
| \node[] (i0j0)  at (-1, -1) {B}; | |
| 
 | |
| \draw[<-] (i0j0) -- (i0j1) node [midway] {$f$}; | |
| \draw[->] (i0j1) -- (i1j1) node [midway] {$g$}; |