diff --git a/Rakefile b/Rakefile
index 031d8d5..8485690 100644
--- a/Rakefile
+++ b/Rakefile
@@ -8,7 +8,7 @@ FileList['src/*.src.md'].each do |file|
   srcfiles << Sec_file.new(file)
 end
 srcfiles = Sec_files.new srcfiles
-srcfiles.renum
+srcfiles.renum!
 
 mdfilenames = srcfiles.map {|srcfile| srcfile.to_md}
 htmlfilenames = srcfiles.map {|srcfile| "html/"+srcfile.to_html}
@@ -129,7 +129,7 @@ task all: [:md, :html, :pdf]
 
 task md: mdfilenames+["Readme.md"]
 
-file "Readme.md" do
+file "Readme.md" => mdfilenames do
   buf = [ "# Gtk4 Tutorial for beginners\n", "\n" ]
   buf << abstract
   buf << "\n"
diff --git a/lib/lib_sec_file.rb b/lib/lib_sec_file.rb
index 6001ece..f315ea3 100644
--- a/lib/lib_sec_file.rb
+++ b/lib/lib_sec_file.rb
@@ -67,7 +67,7 @@ class Sec_file < String
   def is_i?
     self.to_f == self.to_f.floor
   end
-  def renum n
+  def renum! n
     if n.instance_of?(String)
       n = n.to_i if n =~ /^\d+$/
       n = n.to_f if n =~ /^\d+\.\d+/
@@ -99,7 +99,7 @@ class Sec_files < Array
       raise "#{sec_files} is not an array."
     end
   end
-  def renum
+  def renum!
     self.sort!
     tbl = []
     n = 1
@@ -138,7 +138,7 @@ private
           end
         end
         unless found # OK to replace
-          self[i].renum n
+          self[i].renum! n
           tbl[i][2] = true
 #         tbl[0] (old number (String) is kept in the array 'tbl')
           changed = true