Shorthand form with existential type

gistfile1.scala

// Can also do this:
scala> def addBippy(set: EquaSets[String]#EquaSet): set.path.EquaSet = {
     |   set.copyInto(set.path) union set.path.EquaSet("bippy")
     | }
addBippy: (set: org.scalactic.EquaSets[String]#EquaSet)set.path.EquaSet

scala> val lowered = SortedEquaSets[String](StringNormalizations.lowerCased.toOrderingEquality)
lowered: org.scalactic.SortedEquaSets[String] = org.scalactic.SortedEquaSets@26273e89

// But it results in an existential type
scala> addBippy(lowered.EquaSet("hi", "ho"))
<console>:19: warning: inferred existential type set.path.EquaSet forSome { val set: lowered.EquaSet }, which cannot be expressed by wildcards,  should be enabled
by making the implicit value scala.language.existentials visible.
This can be achieved by adding the import clause 'import scala.language.existentials'
or by setting the compiler option -language:existentials.
See the Scala docs for value scala.language.existentials for a discussion
why the feature should be explicitly enabled.
              addBippy(lowered.EquaSet("hi", "ho"))
                      ^
res0: set.path.EquaSet forSome { val set: lowered.EquaSet } = EquaSet(hi, ho, bippy)

scala> import scala.language.existentials
import scala.language.existentials

// Can see it better here
scala> addBippy(lowered.EquaSet("hi", "ho"))
res1: set.path.EquaSet forSome { val set: lowered.EquaSet } = EquaSet(hi, ho, bippy)

// Can "widen" if that's the appropriate word the type back to what it was before
scala> val result: lowered.EquaSet = addBippy(lowered.EquaSet("hi", "ho"))
result: lowered.EquaSet = EquaSet(hi, ho, bippy)