The `godebug` lines in the previous example would be written:

	//go:debug default=go1.21
	//go:debug panicnil=1
	//go:debug asynctimerchan=0

Starting in Go 1.21, the Go toolchain treats a `//go:debug` directive
with an unrecognized GODEBUG setting as an invalid program.
Programs with more than one `//go:debug` line for a given setting
are also treated as invalid.
(Older toolchains ignore `//go:debug` directives entirely.)