title: What's the difference between -- and &~ in Scala
body: <p>'--' the document say: Creates a new $coll from this $coll by removing all elements of another collection.</p>
  <p>'&amp;~' the document say: The difference of this set and another set.</p>
  <p>i use both of two symbol can get same result.</p>
  <p>so is there have some difference between this two symbol? For example, there are different time complexity or memory occupation?</p>