diff --git a/bin/example.rb b/bin/example.rb
index 03787b4..4291c54 100755
--- a/bin/example.rb
+++ b/bin/example.rb
@@ -4,8 +4,9 @@ require 'pathname'
 $:.unshift Pathname.new(__FILE__).dirname.dirname.join('lib').to_s
 require 'dencli'
 
-cli = DenCli.new 'example', "This is an example for generate a DenCli-API"
-cli.cmd( :args, "Expects and prints given arguments", &lambda {|a, b, c:, d:, e:|
+cli = DenCli.new :example, "This is an example for generate a DenCli-API"
+cli.cmd( :args, "Expects and prints given arguments",
+	&lambda {|a, b, c:, d:, e:|
 		p a: a, b: b, c: c, d: d, e: e
 	}).
 	opt( :c, '-c=ForC', "Option c").
diff --git a/lib/dencli.rb b/lib/dencli.rb
index 7681856..c9c8264 100755
--- a/lib/dencli.rb
+++ b/lib/dencli.rb
@@ -1,6 +1,13 @@
 require 'optparse'
 
+
 class DenCli
+	def self.assert_type klass, method, argument_name, object, *types
+		unless types.any? {|type| object.is_a? type }
+			raise ArgumentError, "#{klass.name}.#{method} expects #{types.map( &:to_s).join '|' } for #{argument_name}, not: #{object.inspect}"
+		end
+	end
+
 	class UsageError < ::RuntimeError
 	end
 	class UnknownCommand < UsageError
diff --git a/lib/dencli/cmd.rb b/lib/dencli/cmd.rb
index 12cad0a..1e4bfe1 100644
--- a/lib/dencli/cmd.rb
+++ b/lib/dencli/cmd.rb
@@ -80,8 +80,8 @@ class DenCli::CMD
 		end
 	end
 
-	def commands *args, &exe
-		yield self
+	def commands *args
+		yield name, self
 	end
 
 	def goto *args
diff --git a/lib/dencli/sub.rb b/lib/dencli/sub.rb
index c52ddc7..15207d5 100644
--- a/lib/dencli/sub.rb
+++ b/lib/dencli/sub.rb
@@ -4,6 +4,9 @@ class DenCli::Sub
 	attr_reader :parent, :name, :description, :subs, :aliases
 
 	def initialize parent, name, description
+		#DenCli::assert_type self, __method__, :name, name, Symbol
+		#DenCli::assert_type self, __method__, :parent, parent, DenCli, DenCli::Sub
+		#DenCli::assert_type self, __method__, :description, description, String
 		@parent, @name, @description, @subs, @aliases = parent, name, "-> #{description}", {}, {}
 	end
 
@@ -33,7 +36,7 @@ class DenCli::Sub
 	end
 
 	def commands &exe
-		yield self
+		yield name, self
 		@subs.each {|k, c| c.commands &exe }
 	end
 
@@ -44,8 +47,8 @@ class DenCli::Sub
 	end
 
 	def self._help_commands output, subs
-		m = subs.map {|c| x = c.usage.length; 25 < x ? 0 : x }.max
-		subs.each do |c|
+		m = subs.map {|_name,c| x = c.usage.length; 25 < x ? 0 : x }.max
+		subs.each do |_name, c|
 			if 25 < c.usage.length
 				output << "% -#{m}s\n#{' ' * m} " % [c.usage]
 			else
@@ -96,6 +99,10 @@ class DenCli::Sub
 	end
 
 	def _add name, min, obj, aliases
+		#DenCli::assert_type self, __method__, :name, name, Symbol, NilClass
+		#DenCli::assert_type self, __method__, :min, min, Integer, NilClass
+		#DenCli::assert_type self, __method__, :obj, obj, DenCli::Sub, DenCli::CMD
+		#DenCli::assert_type self, __method__, :aliases, aliases, Array, NilClass
 		name = name.to_s  unless name.nil?
 		@subs[name] = obj
 		DenCli.gen_aliases( name, min) {|a| @aliases[a] = obj }