title: Apply a function to elements inside a list of lists
body: <p>Let's say I have a function:</p>
  
  <pre><code>def f(i: Int): String = i.toString
  </code></pre>
  
  <p>How can I define <strong>xxx</strong> such as (assume I can add methods to list without implicit conversions):</p>
  
  <pre><code>val source1: List[List[Int]] = List(List(1,2), List(3,4))
  val source2: List[Int] = List(1,2)
  
  val result1: List[List[String]] = source1.xxx(f)
  val result2: List[String] = source2.xxx(f)
  </code></pre>
  
  <p>because with </p>
  
  <blockquote>
    <p>List [ _ ]</p>
  </blockquote>
  
  <p>I can't tell if _ can have map applied or not. Is it possible?</p>