### Iterators

The new [`iter` package](/pkg/iter/) provides the basic definitions for

pkg iter, func Pull2[$0 interface{}, $1 interface{}](Seq2[$0, $1]) (func() ($0, $1, bool), func()) #61897
pkg iter, func Pull[$0 interface{}](Seq[$0]) (func() ($0, bool), func()) #61897
pkg iter, type Seq2[$0 interface{}, $1 interface{}] func(func($0, $1) bool) #61897

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

must either all be present or all be absent. If present, each name
stands for one item (parameter or result) of the specified type and
all non-<a href="#Blank_identifier">blank</a> names in the signature
must be <a href="#Uniqueness_of_identifiers">unique</a>.
If absent, each type stands for one item of that type.
Parameter and result
lists are always parenthesized except that if there is exactly

		p.lineNum = p.lex.Line()
		switch tok {
		case '\n', ';':
			continue
		case scanner.EOF:
			return "", "", nil, false
		}
		break
	}
	// First item must be an identifier.
	if tok != scanner.Ident {
		p.errorf("expected identifier, found %q", p.lex.Text())
		return "", "", nil, false // Might as well stop now.
	}
	word, cond = p.lex.Text(), ""

	// The usual case is caught by Push, below, but be safe.
	for nesting := 0; nesting < 100; {
		tok := in.Stack.Next()
		switch tok {
		case '#':
			if !in.beginningOfLine {
				in.Error("'#' must be first item on line")
			}
			in.beginningOfLine = in.hash()
			in.text = "#"
			return '#'

		case scanner.Ident:
			// Is it a macro name?
			name := in.Stack.Text()
			macro := in.macros[name]

must either all be present or all be absent. If present, each name
stands for one item (parameter or result) of the specified type and
all non-<a href="#Blank_identifier">blank</a> names in the signature
must be <a href="#Uniqueness_of_identifiers">unique</a>.
If absent, each type stands for one item of that type.
Parameter and result
lists are always parenthesized except that if there is exactly

the number of items in the channel corresponds to the number of active uses,
the capacity of the channel corresponds to the maximum number of simultaneous uses,
sending an item acquires the semaphore, and receiving an item releases
the semaphore.
This is a common idiom for limiting concurrency.
</p>

<p>
This program starts a goroutine for every entry in the work list, but the

// Package lex implements lexical analysis for the assembler.
package lex

import (
	"fmt"
	"log"
	"os"
	"strings"
	"text/scanner"

	"cmd/internal/src"
)

// A ScanToken represents an input item. It is a simple wrapping of rune, as
// returned by text/scanner.Scanner, plus a couple of extra values.
type ScanToken rune

const (
	// Asm defines some two-character lexemes. We make up

		}
		buf.WriteString(input.Text())
	}
}

type badLexTest struct {
	input string
	error string
}

var badLexTests = []badLexTest{
	{
		"3 #define foo bar\n",
		"'#' must be first item on line",
	},
	{
		"#ifdef foo\nhello",
		"unclosed #ifdef or #ifndef",
	},
	{
		"#ifndef foo\nhello",
		"unclosed #ifdef or #ifndef",
	},
	{
		"#ifdef foo\nhello\n#else\nbye",