title: number several equations with only one number
body: <p>How can I number several equations in a align environment using only one number? </p>
  
  <p>For example</p>
  
  <pre><code>\begin{align}
  w^T x_i + b \geqslant 1-\xi_i \text{ if } y_i=1, \nonumber \\
  w^T x_i + b \leqslant -1+\xi_i \text{ if } y_i=-1,
  \end{align}
  </code></pre>
  
  <p>The numbering will appear next to the second equation. But it would be better if it appears between the lines of the two equations.</p>
  
  <p>In this case how to label this group of equations for later referring to?</p>
  
  <p>Thanks and regards!</p>