\begin{sequencediagram}
  \newthread{t}{:Thread}
  \newinst{a}{:A}
  \newinst{b}{:B}

  \begin{call}{t}{funcA()}{a}{return}
    \begin{call}{a}{funcA()}{b}{return}
    \end{call}
  \end{call}
\end{sequencediagram}