Horizontal alignment of long equations

Question

I hope that the following question is not a duplicate but I could not find a definite answer.

I'm trying to obtain the the same result as Formula (1) in the picture next, but I feel that my approach is not optimal and somewhat dirty.

I produced the output with the following code:

\documentclass{scrartcl}

\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{aligned}
X & = \{ x \mid \text{a very very very long condition} \} \cup \\
  & \phantom{{}={}} \{ y \mid \text{another very very very long condition} \}
\end{aligned}
\end{equation}

\begin{equation}
\begin{split}
X & = \{ x \mid \text{a very very very long condition} \} \cup \\
& \quad \{ y \mid \text{another very very very long condition} \}
\end{split}
\end{equation}

\begin{equation}
\begin{aligned}
X = & \{ x \mid \text{a very very very long condition} \} \cup \\
  & \{ y \mid \text{another very very very long condition} \}
\end{aligned}
\end{equation}

\begin{equation}
X = 
\begin{array}[t]{l}
  \{ x \mid \text{a very very very long condition} \} \cup \\
  \{ y \mid \text{another very very very long condition} \}
\end{array}
\end{equation}

\end{document}

Formula (1) is the output I desire. However, I can't imagine that using \phantom is the right way to go.

Formula (2) is according to the examples in the amsmath package documentation. And it looks ugly.

Formula (3) is better, but the kerning is wrong. However, I somehow feel that using the alignment in this way is the right approach.

Finally, Formula (4) uses an array, but also here the kerning is screwed.

Can anyone hint me in the right direction? Thanx!

Answer 1

In these cases there is no need at all for \phantom commands.

To ensure the correct spacing when binary relations or operators are involved, it suffices to use an empty group {}.

Infact, the code

\documentclass{scrartcl}
\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{aligned}
X  = {} & \{ x \mid \text{a very very very long condition} \} \cup {} \\            
        & \{ y \mid \text{another very very very long condition} \}
\end{aligned}
\end{equation}

\end{document} 

gives the desired spacing: