# Guess a possible nit_dir.
	#
	# It uses, in order:
	#
	# * the option `opt_nit_dir`
	# * the environment variable `NIT_DIR`
	# * the runpath of the program from argv[0]
	# * the runpath of the process from /proc
	# * the search in PATH
	#
	# If there is errors (e.g. the indicated path is invalid) or if no
	# path is found, then an error is displayed and the program exits.
	#
	# The result is returned without being assigned to `nit_dir`.
	# This function is automatically called by `process_options`
	fun locate_nit_dir: String
	do
		# the option has precedence
		var res = opt_nit_dir.value
		if res != null then
			if not check_nit_dir(res) then
				fatal_error(null, "Fatal Error: the value of --nit-dir does not seem to be a valid base Nit directory: {res}.")
			end
			return res
		end

		# then the environ variable has precedence
		res = "NIT_DIR".environ
		if not res.is_empty then
			if not check_nit_dir(res) then
				fatal_error(null, "Fatal Error: the value of NIT_DIR does not seem to be a valid base Nit directory: {res}.")
			end
			return res
		end

		# find the runpath of the program from argv[0]
		res = "{sys.program_name.dirname}/.."
		if check_nit_dir(res) then return res.simplify_path

		# find the runpath of the process from /proc
		var exe = "/proc/self/exe"
		if exe.file_exists then
			res = exe.realpath
			res = res.dirname.join_path("..")
			if check_nit_dir(res) then return res.simplify_path
		end

		# search in the PATH
		var path_sep = if is_windows then ";" else ":"
		var ps = "PATH".environ.split(path_sep)
		for p in ps do
			res = p/".."
			if check_nit_dir(res) then return res.simplify_path
		end

		fatal_error(null, "Fatal Error: cannot locate a valid base Nit directory. It is quite unexpected. Try to set the environment variable `NIT_DIR` or to use the `--nit-dir` option.")
		abort
	end