\magnification=\magstep1

$$
{{d\vec{L}}\over{dt}} \quad = \quad \vec{\tau} 
$$

$$
\vec{\tau} \quad = \quad 0  \quad \Longrightarrow \quad 
     {{d\vec{L}}\over{dt}} \quad = \quad 0  \quad \Longrightarrow \quad
       {\vec{L}} \quad = \quad {\rm constant}
$$

\bye

