\documentclass[12pt]{standalone}
\input{head}
\begin{document}
\begin{tikzpicture}\selectlanguage{english}
  \path (0,2) node(A){$a$} (2,2) node(AP){$a$}
  (0,0) node(B){$b$} (2,0) node(BP){$b$};
  \draw[->] (A) -- node[above]{$1_a$} (AP);
  \draw[->] (AP) -- node[right]{$f$} (BP);
  \draw[->] (A) -- node[left]{$f$} (B);
  \draw[->] (B) -- node[below]{$1_b$} (BP);
  \draw[->] (A) -- node[above]{$f$} (BP);
\end{tikzpicture}
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End: