How to use system package for a system of equations that involve literal indexes?

Question

Here is an example of what seems to me like a basic usecase for the system package:

\documentclass{article}
\usepackage{amsmath}
\usepackage{systeme}
\begin{document}
\begin{equation}
\systeme{x_{1}=x_0,x_{2}=x_1}
\end{equation}
\end{document}

The result displays more or less as expected, but with a weird alignment:

But if I want to a literal for an index, this happens:

\begin{equation*}
\systeme{x_{i}=x_0,x_{2}=x_1}
\end{equation*}

Why is this "i" displaying on the left and how can I remove it?

Answer 1

You can use substitutions.

\documentclass{article}
\usepackage{amsmath}
\usepackage{systeme}
\begin{document}
\begin{equation}
\syssubstitute{{a_1}{x_i}{a_2}{x_2}{b}{x_1}}
\systeme{a_1=x_0,a_2=b}
\end{equation}
\end{document}