diff --git a/README.md b/README.md
index b12b9e6..4426280 100644
--- a/README.md
+++ b/README.md
@@ -59,7 +59,6 @@ and to make it a proxy editor for each of the URLs in the list.
 ```
 vivosnap.rb prepare sub-list [session_list_file] [count] [sub_list_file]
 ```
-**NOT IMPLEMENTED**  
 Create a smaller session list from an existing one. 
 The new list will have the specified number of entries, extracted at even intervals from the existing list.
 
diff --git a/cmd_prepare_self_editor_account/cmd_prepare_self_editor_account.rb b/cmd_prepare_self_editor_account/cmd_prepare_self_editor_account.rb
index e558345..2de5652 100644
--- a/cmd_prepare_self_editor_account/cmd_prepare_self_editor_account.rb
+++ b/cmd_prepare_self_editor_account/cmd_prepare_self_editor_account.rb
@@ -112,6 +112,7 @@ class CmdPrepareSelfEditorAccount
   AUTH_SELF_EDITOR = RDF::URI.new('http://vitro.mannlib.cornell.edu/ns/vitro/authorization#SELF_EDITOR')
   AUTH_STATUS = RDF::URI.new('http://vitro.mannlib.cornell.edu/ns/vitro/authorization#status')
   AUTH_USER_ACCOUNT = RDF::URI.new('http://vitro.mannlib.cornell.edu/ns/vitro/authorization#UserAccount')
+
   def initialize(args)
     @args = args
 
diff --git a/cmd_prepare_session_list/cmd_prepare_session_list.rb b/cmd_prepare_session_list/cmd_prepare_session_list.rb
index 64dea4b..49e1c05 100644
--- a/cmd_prepare_session_list/cmd_prepare_session_list.rb
+++ b/cmd_prepare_session_list/cmd_prepare_session_list.rb
@@ -22,6 +22,7 @@
 
 class CmdPrepareSessionList
   USAGE = 'prepare session-list [uri_list_file] {account_email:account_password} {session_list_file {REPLACE}}'
+  
   def initialize(args)
     @args = args
     @replace = true && args.delete('REPLACE')
@@ -47,7 +48,7 @@ def parse_remaining_args(args)
       [split_credentials(args[1]), args[2]]
     end
   end
-  
+
   def split_credentials(arg)
     complain("usage: #{USAGE}") unless 1 == arg.count(':')
     arg.split(':')
diff --git a/cmd_prepare_sub_list/cmd_prepare_sub_list.rb b/cmd_prepare_sub_list/cmd_prepare_sub_list.rb
index 247501b..641d1e3 100644
--- a/cmd_prepare_sub_list/cmd_prepare_sub_list.rb
+++ b/cmd_prepare_sub_list/cmd_prepare_sub_list.rb
@@ -1,10 +1,70 @@
+=begin
+--------------------------------------------------------------------------------
+
+Create a sub-list sample from a session list file.
+
+You provide the session list file, and how many sessions you want to put into
+the sub-list, and the tool will extract that many session lines from the original
+list, evenly spaced, and put them into the new file.
+
+--------------------------------------------------------------------------------
+
+vivosnap.rb prepare sub-list [session_list_file] [count] [sub_list_file]
+
+--------------------------------------------------------------------------------
+=end
+
 class CmdPrepareSubList
+  include ::ArgsChecker
+
+  USAGE = "prepare sub-list [session_list_file] [count] {sub_list_file {REPLACE}}"
   def initialize(args)
-    bogus "CmdPrepareSubList.initialize NOT IMPLEMENTED"
+    @args = args
+    @replace = args.delete('REPLACE')
+
+    complain("usage: #{USAGE}") unless (2..3).include? args.size
+
+    @session_list_file = confirm_file_exists(args[0])
+    @line_count = confirm_positive_integer(args[1])
+    @output = set_output_io(args[2], @replace)
   end
-  
+
   def run()
-    bogus "CmdPrepareSubList.run NOT IMPLEMENTED"
+    read_lines_and_remove_comments
+    complain("#{@session_list_file} only contains #{@lines.size} effective lines.") if @lines.size < @line_count
+
+    write_heading()
+    write_selected_lines()
+    write_report()
   end
+
+  def write_heading()
+    @output.puts "#"
+    @output.puts "# prepare sub-list #{@args.join(' ')}"
+    @output.puts "# #{Time.now.strftime('%Y-%m-%d %H:%M:%S')}"
+    @output.puts "#"
+  end
+
+  def read_lines_and_remove_comments()
+    @lines = File.readlines(@session_list_file)
+    @lines.reject! do |line|
+      line.start_with?('#') || line.strip.empty?
+    end
+  end
+
+  def write_selected_lines()
+    interval = (@lines.size / @line_count).to_i
+    (1..@line_count).each do |i|
+      @output.puts @lines[(i - 1) * interval]
+    end
+  end
+
+  def write_report()
+    report = "#\n#   Wrote #{@line_count} lines.\n#\n"
+
+    @output.print(report)
+    print(report) if @output != $stdout
+  end
+
 end
 
diff --git a/cmd_prepare_uri_list/cmd_prepare_uri_list.rb b/cmd_prepare_uri_list/cmd_prepare_uri_list.rb
index a06fd62..973b5c4 100644
--- a/cmd_prepare_uri_list/cmd_prepare_uri_list.rb
+++ b/cmd_prepare_uri_list/cmd_prepare_uri_list.rb
@@ -7,14 +7,21 @@
 write the results to the file, one URI per line.
 
 --------------------------------------------------------------------------------
+
+vivosnap.rb prepare uri-list [class_list_file] [VIVO_homepage_URL] {uri_list_file {REPLACE}}
+
+--------------------------------------------------------------------------------
+
 =end
 
 class CmdPrepareUriList
+  include ::ArgsChecker
+
   USAGE = 'prepare uri-list [class_list_file] [VIVO_homepage_URL] {uri_list_file {REPLACE}}'
   def initialize(args)
     @args = args
     @replace = args.delete('REPLACE')
-    
+
     complain("usage: #{USAGE}") unless (2..3).include? args.size
 
     @class_list_file = confirm_file_exists(args[0])
diff --git a/utils/args_checker.rb b/utils/args_checker.rb
index 2ec17d7..e3fdc00 100644
--- a/utils/args_checker.rb
+++ b/utils/args_checker.rb
@@ -30,7 +30,7 @@ def confirm_vivo_home_url(url)
     rescue
       complain("Can't contact VIVO at '#{url}': #{$!}")
     end
-    
+
     url
   end
 
@@ -49,4 +49,17 @@ def set_output_io(path, replace)
       $stdout
     end
   end
+
+  #
+  # The parameter should be a String representing an integer greater than 0.
+  #
+  def confirm_positive_integer(count_str)
+    begin
+      count = count_str.to_i
+    rescue
+      complain("Not a valid integer: '#{count_str}'")
+    end
+    complain("Expecting a positive integer, not '#{count_str}'") unless count > 0
+    count
+  end
 end
\ No newline at end of file