foo $host;
foo $geoip_city_country_code3
set $0 foo;
set $_ foo;
set $arg_? foo;

# real example

log_format main      '$remote_addr - $remote_user [$time_local]  '
  '"$request" $status $bytes_sent '
  '"$http_referer" "$http_user_agent" '
  '"$gzip_ratio"';

location / {
    ssi on;
    set $inc $request_uri;
    if (!-f $request_filename) {
        rewrite ^ /index.html last;
    }
    if (!-f $document_root$inc.html) {
        return 404;
    }
}

----------------------------------------------------

[
	["directive", [
		["keyword", "foo"],
		["variable", "$host"]
	]],
	["punctuation", ";"],

	["directive", [
		["keyword", "foo"], ["variable", "$geoip_city_country_code3"],
		"\r\nset ", ["variable", "$0"], " foo"
	]],
	["punctuation", ";"],

	["directive", [
		["keyword", "set"],
		["variable", "$_"],
		" foo"
	]],
	["punctuation", ";"],

	["directive", [
		["keyword", "set"],
		["variable", "$arg_?"],
		" foo"
	]],
	["punctuation", ";"],

	["comment", "# real example"],

	["directive", [
		["keyword", "log_format"],
		" main      ",
		["string", [
			"'",
			["variable", "$remote_addr"],
			" - ",
			["variable", "$remote_user"],
			" [",
			["variable", "$time_local]"],
			"  '"
		]],

		["string", [
			"'\"",
			["variable", "$request"],
			"\" ",
			["variable", "$status"],
			["variable", "$bytes_sent"],
			" '"
		]],

		["string", [
			"'\"",
			["variable", "$http_referer"],
			"\" \"",
			["variable", "$http_user_agent"],
			"\" '"
		]],

		["string", [
			"'\"",
			["variable", "$gzip_ratio"],
			"\"'"
		]]
	]],
	["punctuation", ";"],

	["directive", [
		["keyword", "location"],
		" /"
	]],
	["punctuation", "{"],

	["directive", [
		["keyword", "ssi"],
		["boolean", "on"]
	]],
	["punctuation", ";"],

	["directive", [
		["keyword", "set"],
		["variable", "$inc"],
		["variable", "$request_uri"]
	]],
	["punctuation", ";"],

	["directive", [
		["keyword", "if"],
		" (!-f ",
		["variable", "$request_filename"],
		")"
	]],
	["punctuation", "{"],

	["directive", [
		["keyword", "rewrite"],
		" ^ /index.html last"
	]],
	["punctuation", ";"],

	["punctuation", "}"],

	["directive", [
		["keyword", "if"],
		" (!-f ",
		["variable", "$document_root"],
		["variable", "$inc"],
		".html)"
	]],
	["punctuation", "{"],

	["directive", [
		["keyword", "return"],
		["number", "404"]
	]],
	["punctuation", ";"],

	["punctuation", "}"],

	["punctuation", "}"]
]

----------------------------------------------------

Checks for variables.
