### Iterators

The new [iter] package provides the basic definitions for working with
user-defined iterators.

The [slices] package adds several functions that work with iterators:
- [All](/pkg/slices#All) returns an iterator over slice indexes and values.
- [Values](/pkg/slices#Values) returns an iterator over slice elements.
- [Backward](/pkg/slices#Backward) returns an iterator that loops over
  a slice backward.

pkg maps, func Collect[$0 comparable, $1 interface{}](iter.Seq2[$0, $1]) map[$0]$1 #61900
pkg maps, func Insert[$0 interface{ ~map[$1]$2 }, $1 comparable, $2 interface{}]($0, iter.Seq2[$1, $2]) #61900
pkg maps, func Keys[$0 interface{ ~map[$1]$2 }, $1 comparable, $2 interface{}]($0) iter.Seq[$1] #61900
pkg maps, func Values[$0 interface{ ~map[$1]$2 }, $1 comparable, $2 interface{}]($0) iter.Seq[$2] #61900

<!-- see ../../3-iter.md -->...

    Iterator<Entry<String, String>> iter = copy.entrySet().iterator();
    assertTrue(iter.hasNext());
    Entry<String, String> entry = iter.next();
    assertEquals("Hello", entry.getKey());
    assertEquals("World", entry.getValue());
    assertTrue(iter.hasNext());

    entry = iter.next();
    assertEquals("first", entry.getKey());

// If a name is in this map, it is allowed only in listed packages,
// even if it has a linknamed definition.
var blockedLinknames = map[string][]string{
	// coroutines
	"runtime.coroswitch": {"iter"},
	"runtime.newcoro":    {"iter"},
	// weak references
	"internal/weak.runtime_registerWeakPointer": {"internal/weak"},
	"internal/weak.runtime_makeStrongFromWeak":  {"internal/weak"},
}

				runTests: "SystemRoots",
				env:      []string{"GOOS=ios", "CGO_ENABLED=1"},
				pkg:      "crypto/x509",
			})
	}

	// GOEXPERIMENT=rangefunc tests
	if !t.compileOnly {
		for _, pkg := range []string{"iter", "slices", "maps"} {
			t.registerTest("GOEXPERIMENT=rangefunc",
				&goTest{
					variant: pkg,
					short:   t.short,
					env:     []string{"GOEXPERIMENT=rangefunc"},
					pkg:     pkg,
				})
		}