Avoid method in method

And rename `File.concat_path` to `File._concat_path`.

mrbgems/mruby-io/mrblib/file.rb

@@ -55,46 +55,46 @@ class File < IO
     s
   end
 
-  def self.expand_path(path, default_dir = '.')
-    def concat_path(path, base_path)
-      if path[0] == "/" || path[1] == ':' # Windows root!
-        expanded_path = path
-      elsif path[0] == "~"
-        if (path[1] == "/" || path[1] == nil)
-          dir = path[1, path.size]
-          home_dir = _gethome
-
-          unless home_dir
-            raise ArgumentError, "couldn't find HOME environment -- expanding '~'"
-          end
-
-          expanded_path = home_dir
-          expanded_path += dir if dir
-          expanded_path += "/"
-        else
-          splitted_path = path.split("/")
-          user = splitted_path[0][1, splitted_path[0].size]
-          dir = "/" + splitted_path[1, splitted_path.size].join("/")
+  def self._concat_path(path, base_path)
+    if path[0] == "/" || path[1] == ':' # Windows root!
+      expanded_path = path
+    elsif path[0] == "~"
+      if (path[1] == "/" || path[1] == nil)
+        dir = path[1, path.size]
+        home_dir = _gethome
+
+        unless home_dir
+          raise ArgumentError, "couldn't find HOME environment -- expanding '~'"
+        end
 
-          home_dir = _gethome(user)
+        expanded_path = home_dir
+        expanded_path += dir if dir
+        expanded_path += "/"
+      else
+        splitted_path = path.split("/")
+        user = splitted_path[0][1, splitted_path[0].size]
+        dir = "/" + splitted_path[1, splitted_path.size].join("/")
 
-          unless home_dir
-            raise ArgumentError, "user #{user} doesn't exist"
-          end
+        home_dir = _gethome(user)
 
-          expanded_path = home_dir
-          expanded_path += dir if dir
-          expanded_path += "/"
+        unless home_dir
+          raise ArgumentError, "user #{user} doesn't exist"
         end
-      else
-        expanded_path = concat_path(base_path, _getwd)
-        expanded_path += "/" + path
-      end
 
-      expanded_path
+        expanded_path = home_dir
+        expanded_path += dir if dir
+        expanded_path += "/"
+      end
+    else
+      expanded_path = _concat_path(base_path, _getwd)
+      expanded_path += "/" + path
     end
 
-    expanded_path = concat_path(path, default_dir)
+    expanded_path
+  end
+
+  def self.expand_path(path, default_dir = '.')
+    expanded_path = _concat_path(path, default_dir)
     drive_prefix = ""
     if File::ALT_SEPARATOR && expanded_path.size > 2 &&
         ("A".."Z").include?(expanded_path[0].upcase) && expanded_path[1] == ":"