It is only safe to forget

Safe and subtyping.scala

def Environment[A, X](x: X) = {
  val a: A = x
  // this is safe iff X <: A
  // in other words, if we discard information 
  // (otherwise we might try to use `a` in an operation it doesn't support)
  
  // in other words: it is only safe to forget information
  
  // another form:
  def f(a: A) = {
    // ...
  }
  
  f(x)  
  // again only safe if X <: A
  // inside f, only information available in A can be used.
  
  // another form:
  def g: A = x
  
}