\begin{tabbing}
I(\=$v$) where I($\alpha$) = \+
\\[0ex]wh\=en $x$ = $\alpha$ $<$ 0,  $y$ = I($\alpha$+1).\+
\\[0ex]$d$($x$;$y$)
\-\\[0ex]when $\alpha$ = 0.  $b$when  $w$ = $\alpha$ $>$ 0,  $z$ = I($\alpha${-}1).  $u$($w$;$z$)end where 
\-\\[0ex]\textbf{is Primitive}
\end{tabbing}