title: Scala variable naming rules with underscore
body: <p>What are the rules to name methods and variables in Scala, especially when mixing symbols and letters using <code>_</code>? For instance, why <code>_a_</code>, <code>a_+</code>, <code>__a</code>, <code>__a__a__a__+</code>, <code>___</code> are valid names, but <code>_a_+_a</code> or <code>_a_+_</code> are not?</p>