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.
220.127.116.11.5.1 Examples of for-as-across subclause