Switching between leqno and reqno

Using amsmath package, the equation numbering is by default set to right, i.e., reqno. It is possible to change to leqno by \documentclass[leqno,...]{article}, for example. However, if one desires to temporarily switch between the two a good solution can be found at

http://www.golatex.de/wechsel-zwischen-leqno-und-reqno-fleqn-uvm-t2488.html

For convenience, I paste Herbert's code here:

\makeatletter 
\def\Links{\tagsleft@true}\def\Rechts{\tagsleft@false} 
\makeatother 

Examples of use:

\begin{align}
y=f(x) 
\end{align} 

\Links 
\begin{align}
y=f(x) 
\end{align} 

\Rechts 
\begin{align}
y=f(x) 
\end{align}