repl.swift:1:22: error: value of type 'Zip2Generator<IndexingGenerator<Array<Int>>, IndexingGenerator<Array<Int>>>' has no member 'next'
var gen = zip([1,2,3],[7,8,9]).generate()
next() mutates the object that it is called on, so that object must be assigned to a
var for it to work.
zip([1,2,3],[7,8,9]).generate() is immutable until you assign it to a
next() mutate the object? Because there is an internal index variable that is keeping track of which value to show next.
next() uses that internal index and then advances it (mutates it) to point to the next item.