diff --git a/lib/core.rb b/lib/core.rb
index 86c9f7f..e50a916 100644
--- a/lib/core.rb
+++ b/lib/core.rb
@@ -5,6 +5,7 @@ require_relative './language/mode'
 require_relative './language/operations'
 require_relative './language/program'
 require_relative './language/stack'
+require_relative './language/string'
 require_relative './language/time-date'
 
 module Rpl
@@ -33,7 +34,7 @@ module Rpl
         args << elt
       end
 
-      [stack, args.reverse]
+      [stack, args]
     end
 
     def __todo( stack )
diff --git a/lib/dictionary.rb b/lib/dictionary.rb
index c24cefe..569a25b 100644
--- a/lib/dictionary.rb
+++ b/lib/dictionary.rb
@@ -94,13 +94,13 @@ module Rpl
       add( 'same',    proc { |stack| Rpl::Core.__todo( stack ) } ) # boolean operator same (equal)
 
       # STRING
-      add( '->str',   proc { |stack| Rpl::Core.__todo( stack ) } ) # convert an object into a string
+      add( '->str',   proc { |stack| Rpl::Core.to_string( stack ) } )
       add( 'str->',   proc { |stack| Rpl::Core.__todo( stack ) } ) # convert a string into an object
       add( 'chr',     proc { |stack| Rpl::Core.__todo( stack ) } ) # convert ASCII character code in stack level 1 into a string
       add( 'num',     proc { |stack| Rpl::Core.__todo( stack ) } ) # return ASCII code of the first character of the string in stack level 1 as a real number
-      add( 'size',    proc { |stack| Rpl::Core.__todo( stack ) } ) # return the length of the string
+      add( 'size',    proc { |stack| Rpl::Core.size( stack ) } )
       add( 'pos',     proc { |stack| Rpl::Core.__todo( stack ) } ) # seach for the string in level 1 within the string in level 2
-      add( 'sub',     proc { |stack| Rpl::Core.__todo( stack ) } ) # return a substring of the string in level 3
+      add( 'sub',     proc { |stack| Rpl::Core.sub( stack ) } )
 
       # BRANCH
       add( 'if',      proc { |stack| Rpl::Core.__todo( stack ) } ) # if <test-instruction> then <true-instructions> else <false-instructions> end
diff --git a/lib/language/operations.rb b/lib/language/operations.rb
index bb1f8c1..b67ef72 100644
--- a/lib/language/operations.rb
+++ b/lib/language/operations.rb
@@ -7,13 +7,13 @@ module Rpl
       addable = %i[numeric string name]
       stack, args = Rpl::Core.stack_extract( stack, [addable, addable] )
 
-      result = { type: case args[0][:type]
+      result = { type: case args[1][:type]
                        when :name
                          :name
                        when :string
                          :string
                        when :numeric
-                         if args[1][:type] == :numeric
+                         if args[0][:type] == :numeric
                            :numeric
                          else
                            :string
@@ -26,11 +26,11 @@ module Rpl
 
       result[:value] = case result[:type]
                        when :name
-                         "'#{args[0][:value]}#{args[1][:value]}'"
+                         "'#{args[1][:value]}#{args[0][:value]}'"
                        when :string
-                         "\"#{args[0][:value]}#{args[1][:value]}\""
+                         "\"#{args[1][:value]}#{args[0][:value]}\""
                        when :numeric
-                         args[0][:value] + args[1][:value]
+                         args[1][:value] + args[0][:value]
                        end
 
       result[:base] = 10 if result[:type] == :numeric # TODO: what if operands have other bases ?
@@ -43,7 +43,7 @@ module Rpl
       stack, args = Rpl::Core.stack_extract( stack, [%i[numeric], %i[numeric]] )
 
       stack << { type: :numeric, base: 10,
-                 value: args[0][:value] - args[1][:value] }
+                 value: args[1][:value] - args[0][:value] }
     end
 
     # negation
@@ -59,7 +59,7 @@ module Rpl
       stack, args = Rpl::Core.stack_extract( stack, [%i[numeric], %i[numeric]] )
 
       stack << { type: :numeric, base: 10,
-                 value: args[0][:value] * args[1][:value] }
+                 value: args[1][:value] * args[0][:value] }
     end
 
     # division
@@ -69,7 +69,7 @@ module Rpl
       raise 'Division by 0' if args[0][:value].zero?
 
       stack << { type: :numeric, base: 10,
-                 value: args[0][:value] / args[1][:value] }
+                 value: args[1][:value] / args[0][:value] }
     end
 
     # inverse
@@ -87,7 +87,7 @@ module Rpl
       stack, args = Rpl::Core.stack_extract( stack, [%i[numeric], %i[numeric]] )
 
       stack << { type: :numeric, base: 10,
-                 value: args[0][:value]**args[1][:value] }
+                 value: args[1][:value]**args[0][:value] }
     end
 
     # rpn_square root
@@ -133,9 +133,9 @@ module Rpl
     def base( stack )
       stack, args = Rpl::Core.stack_extract( stack, [%i[numeric], %i[numeric]] )
 
-      args[0][:base] = args[1][:value]
+      args[1][:base] = args[0][:value]
 
-      stack << args[0]
+      stack << args[1]
     end
 
     # 1 if number at stack level 1 is > 0, 0 if == 0, -1 if <= 0
diff --git a/lib/language/stack.rb b/lib/language/stack.rb
index 084e20d..e912c39 100644
--- a/lib/language/stack.rb
+++ b/lib/language/stack.rb
@@ -6,7 +6,7 @@ module Rpl
     def swap( stack )
       stack, args = Rpl::Core.stack_extract( stack, %i[any any] )
 
-      stack << args[1] << args[0]
+      stack << args[0] << args[1]
     end
 
     # drop first stack entry
@@ -36,7 +36,7 @@ module Rpl
     def rot( stack )
       stack, args = Rpl::Core.stack_extract( stack, %i[any any any] )
 
-      stack << args[1] << args[2] << args[0]
+      stack << args[1] << args[0] << args[2]
     end
 
     # duplicate first stack entry
@@ -55,6 +55,8 @@ module Rpl
       n = args[0][:value]
       stack, args = Rpl::Core.stack_extract( stack, %i[any] * args[0][:value] )
 
+      args.reverse!
+
       2.times do
         n.times.each do |i|
           stack << args[ i ]
@@ -70,6 +72,8 @@ module Rpl
       n = args[0][:value]
       stack, args = Rpl::Core.stack_extract( stack, %i[any] * args[0][:value] )
 
+      args.reverse!
+
       n.times.each do |i|
         stack << args[ i ]
       end
@@ -89,6 +93,8 @@ module Rpl
       n = args[0][:value]
       stack, args = Rpl::Core.stack_extract( stack, %i[any] * args[0][:value] )
 
+      args.reverse!
+
       (1..(n - 1)).each do |i|
         stack << args[ i ]
       end
@@ -103,6 +109,8 @@ module Rpl
       n = args[0][:value]
       stack, args = Rpl::Core.stack_extract( stack, %i[any] * args[0][:value] )
 
+      args.reverse!
+
       stack << args[n - 1]
 
       (0..(n - 2)).each do |i|
diff --git a/lib/language/string.rb b/lib/language/string.rb
new file mode 100644
index 0000000..e5549ac
--- /dev/null
+++ b/lib/language/string.rb
@@ -0,0 +1,31 @@
+module Rpl
+  module Core
+    module_function
+
+    # convert an object into a string
+    def to_string( stack )
+      stack, args = Rpl::Core.stack_extract( stack, :any )
+
+      stack << { type: :string,
+                 value: args[0][:value].to_s }
+    end
+
+    # return the length of the string
+    def size( stack )
+      stack, args = Rpl::Core.stack_extract( stack, %i[string] )
+
+      stack << { type: :numeric,
+                 base: 10,
+                 value: args[0][:value].length }
+    end
+
+    # return a substring of the string in level 3
+    def sub( stack )
+      stack, args = Rpl::Core.stack_extract( stack, [%i[numeric], %i[numeric], %i[string]] )
+
+      puts "#{args[0][:value]}[#{args[1][:value]}..#{args[2][:value]}]"
+      stack << { type: :string,
+                 value: args[2][:value][args[1][:value]..args[0][:value]] }
+    end
+  end
+end
diff --git a/spec/language_string_spec.rb b/spec/language_string_spec.rb
new file mode 100644
index 0000000..bb04988
--- /dev/null
+++ b/spec/language_string_spec.rb
@@ -0,0 +1,17 @@
+# coding: utf-8
+# frozen_string_literal: true
+
+require 'test/unit'
+
+require_relative '../lib/core'
+
+class TestLanguageString < Test::Unit::TestCase
+  def test_sub
+    stack = Rpl::Core.sub( [{ value: 'test', type: :string },
+                            { value: 1, type: :numeric, base: 10 },
+                            { value: 2, type: :numeric, base: 10 }] )
+
+    assert_equal [{ value: 'es', type: :string }],
+                 stack
+  end
+end