In the for-as-across subclause the for 
  or as construct binds the variable var to the value of
  each element in the array vector.
  The loop keyword across marks the array vector; across
  is used as a preposition in this syntax.
  Iteration stops when there are no more elements in the supplied
  array that can be referenced.
  Some implementations might recognize a the special form
  in the vector form to produce more efficient code.