I would like to show a bunch of equations can imply another bunch of equations, with double right arrow in between. Here are two trials of mine: one is to use another level of math mode to enclose the two groups of equations, and the other is to use a table with each group of equation being in a cell.

The first one

```
$$
\begin{align*}
\nabla f(x) -\mu X^{-1} e + A(x) \lambda - z = 0, 
\\ c(x) = 0, 
\\ -\mu X^{-1} e + z = 0, 
\\ x > 0
\\ z > 0
\end{align*} 
\quad \Longrightarrow \quad
\begin{align*}
\nabla f(x) + A(x) \lambda - 2z = 0, 
\\ c(x) = 0, 
\\ -\mu X^{-1} e + z = 0, 
\\ x > 0
\\ z > 0
\\
\end{align*}
$$

```

This one will leads to error 

> 
> amsmath: Erroneous nesting of equation structures;(amsmath) trying to
>  recover with `aligned'. \end{align\*}
> 
> 
> 

Another is using `tabular`:

```
\begin{tabular*}{c c c}
\begin{align*}
\nabla f(x) -\mu X^{-1} e + A(x) \lambda - z = 0, 
\\ c(x) = 0, 
\\ -\mu X^{-1} e + z = 0, 
\\ x > 0
\\ z > 0
\end{align*} 
& $\Longrightarrow$ &
\begin{align*}
\nabla f(x) + A(x) \lambda - 2z = 0, 
\\ c(x) = 0, 
\\ -\mu X^{-1} e + z = 0, 
\\ x > 0
\\ z > 0
\\
\end{align*}
\end{tabular*}

```

It doesn't compile either.

I wonder how to make the two approaches workable, and if there are other better ways?