[runtime/metrics](/pkg/runtime/metrics/) counter
named `/godebug/non-default-behavior/<name>:events`
that counts the number of times a particular program's
behavior has changed based on a non-default value
for that setting.
For example, when `GODEBUG=http2client=0` is set,
`/godebug/non-default-behavior/http2client:events`
counts the number of HTTP transports that the program
has configured without HTTP/2 support.

## Default GODEBUG Values {#default}

// input as a sequence of space-delimited tokens.
func ExampleScanner_words() {
	// An artificial input source.
	const input = "Now is the winter of our discontent,\nMade glorious summer by this sun of York.\n"
	scanner := bufio.NewScanner(strings.NewReader(input))
	// Set the split function for the scanning operation.
	scanner.Split(bufio.ScanWords)
	// Count the words.
	count := 0
	for scanner.Scan() {

+ If there is a corresponding issue, add either `Fixes #1234` or `Updates #1234`
  (the latter if this is not a complete fix) to this comment
+ If referring to a repo other than `golang/go` you can use the
  `owner/repo#issue_number` syntax: `Fixes golang/tools#1234`
+ We do not use Signed-off-by lines in Go. Please don't add them.
  Our Gerrit server & GitHub bots enforce CLA compliance instead.

	{"3*(2+3)", 3 * (2 + 3), true},
	{"3*-(2+3)", 3 * -(2 + 3), true},
	{"3<<2+4", 3<<2 + 4, true},
	{"3<<2+4", 3<<2 + 4, true},
	{"3<<(2+4)", 3 << (2 + 4), true},
	// Junk at EOF.
	{"3 x", 3, false},
	// Big number
	{"4611686018427387904", 4611686018427387904, true},
}

func TestExpr(t *testing.T) {
	p := NewParser(nil, nil, nil) // Expression evaluation uses none of these fields of the parser.
	for i, test := range exprTests {

	// RegisterNumber converts R(10) into arm.REG_R10.
	RegisterNumber func(string, int16) (int16, bool)
	// Instruction is a jump.
	IsJump func(word string) bool
}

// nilRegisterNumber is the register number function for architectures
// that do not accept the R(N) notation. It always returns failure.
func nilRegisterNumber(name string, n int16) (int16, bool) {
	return 0, false
}

// input. The arguments are an initial substring of the remaining unprocessed
// data and a flag, atEOF, that reports whether the [Reader] has no more data
// to give. The return values are the number of bytes to advance the input
// and the next token to return to the user, if any, plus an error, if any.
//
// Scanning stops if the function returns an error, in which case some of

// The len built-in function returns the length of v, according to its type:
//
//	Array: the number of elements in v.
//	Pointer to array: the number of elements in *v (even if v is nil).
//	Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
//	String: the number of bytes in v.
//	Channel: the number of elements queued (unread) in the channel buffer;
//	         if v is nil, len(v) is zero.
//

except.txt lists features that may disappear without breaking true
compatibility.

Starting with go1.19.txt, each API feature line must end in "#nnnnn"
giving the GitHub issue number of the proposal issue that accepted
the new API. This helps with our end-of-cycle audit of new APIs.
The same requirement applies to next/* (described below), which will
become a go1.XX.txt for XX >= 19.

The [Repeat] function returns a new slice that repeats the

The [RuneLen] function returns the number of 16-bit words in
the UTF-16 encoding of the rune. It returns -1 if the rune