diff --git a/lib/ruby_algebra/command.rb b/lib/ruby_algebra/command.rb
index b6ad03c..c9f517e 100644
--- a/lib/ruby_algebra/command.rb
+++ b/lib/ruby_algebra/command.rb
@@ -19,19 +19,22 @@ module RubyAlgebra
     def to_s
       result = "Assignment command: "
       result += @lhs.is_a?(Array) ? @lhs.join(', ') : @lhs.to_s
-      result += ' := '
-      result += @operand1.to_s
-      case @operation
-      when :add
-        result += ' + '
-      when :sub
-        result += ' - '
-      when :mult, :scale
-        result += ' * '
-      when :div
-        result += ' / '
+      if @operation == :diff
+        result += ' := Diff(' + @operand1.to_s + @operand2.map { |v| ", #{v}"}.join + ')'
+      else
+        result += ' := ' + @operand1.to_s
+        case @operation
+        when :add
+          result += ' + '
+        when :sub
+          result += ' - '
+        when :mult, :scale
+          result += ' * '
+        when :div
+          result += ' / '
+        end
+        result += @operand2.to_s if @operation != :assign
       end
-      result += @operand2.to_s if @operation != :assign
       result
     end
   end
diff --git a/lib/ruby_algebra/parser.rb b/lib/ruby_algebra/parser.rb
index 656dfa5..80ef029 100644
--- a/lib/ruby_algebra/parser.rb
+++ b/lib/ruby_algebra/parser.rb
@@ -141,7 +141,7 @@ module RubyAlgebra
             token = IdentifierToken.new(@expr[@i...j])
             @i = j
             return token
-          elsif c.match(%r{[+\-*/^():]})
+          elsif c.match(%r{[+\-*/^():,]})
             puts 'operator'
             token = nil
             case c
@@ -159,6 +159,8 @@ module RubyAlgebra
               token = ParenthesisToken.new(false)
             when ')'
               token = ParenthesisToken.new(true)
+            when ','
+              token = CommaToken.new
             end
             @i += 1
             if c == ':' && @expr[@i] == '='
@@ -200,33 +202,68 @@ module RubyAlgebra
       if tokenizer.lookahead.type == :assign
         n = tokenizer.next_token
         operand1 = tokenizer.lookahead
-        unless operand1.type == :id && !operand1.variable?
-          return AssignmentCommand.new(left_hand_side, :assign, parse_polynomial(tokenizer), nil)
-        end
-        tokenizer.next_token
-        operator = tokenizer.next_token
-        raise StandardError, "unexpected token #{operator.type}, expected +, -, * or /" unless [:plus, :minus, :mult, :div].include?(operator.type)
-
-        operand2 = tokenizer.next_token
-        unless ([:plus, :minus, :mult].include?(operator.type) && operand2.type == :id && !operand2.variable?) ||
-               ([:mult, :div].include?(operator.type) && operand2.type == :num)
-          raise StandardError, 'unsupported operation or invalid syntax'
-        end
-        if operand2.type == :num
-          if operator.type == :mult
-            return AssignmentCommand.new(left_hand_side, :scale, operand1.symbol, operand2.value)
+        if operand1.type == :id && !operand1.variable?
+          if operand1.symbol == 'Diff'
+            tokenizer.next_token
+            n = tokenizer.next_token
+            unless n.type == :paren && n.closing == false
+              raise StandardError, "unexpected token #{n.type}, expected ("
+            end
+            n = tokenizer.lookahead
+            if n.type == :id && !n.variable?
+              tokenizer.next_token
+              target = n.symbol
+            else
+              target = parse_polynomial(tokenizer)
+            end
+            diff_variables = []
+            while tokenizer.lookahead.type == :comma
+              tokenizer.next_token
+              n = tokenizer.next_token
+              unless n.type == :id && n.variable?
+                raise StandardError, "unexpected token #{n.type}, expected variable"
+              end
+              diff_variables << n.symbol
+            end
+            n = tokenizer.next_token
+            unless n.type == :paren && n.closing == true
+              raise StandardError, "unexpected token #{n.type}, expected )"
+            end
+            unless tokenizer.next_token.type == :end
+              raise StandardError, "unexpected token at the end"
+            end
+            return AssignmentCommand.new(left_hand_side, :diff, target, diff_variables)
+          elsif operand1.symbol == 'Subs'
+            raise NotImplementedError
           else
-            return AssignmentCommand.new(left_hand_side, :scale, operand1.symbol, 1.0 / operand2.value)
-          end
-        elsif operand2.type == :id
-          case operator.type
-          when :plus
-            return AssignmentCommand.new(left_hand_side, :add, operand1.symbol, operand2.symbol)
-          when :minus
-            return AssignmentCommand.new(left_hand_side, :sub, operand1.symbol, operand2.symbol)
-          when :mult
-            return AssignmentCommand.new(left_hand_side, :mult, operand1.symbol, operand2.symbol)
+            tokenizer.next_token
+            operator = tokenizer.next_token
+            raise StandardError, "unexpected token #{operator.type}, expected +, -, * or /" unless [:plus, :minus, :mult, :div].include?(operator.type)
+
+            operand2 = tokenizer.next_token
+            unless ([:plus, :minus, :mult].include?(operator.type) && operand2.type == :id && !operand2.variable?) ||
+                  ([:mult, :div].include?(operator.type) && operand2.type == :num)
+              raise StandardError, 'unsupported operation or invalid syntax'
+            end
+            if operand2.type == :num
+              if operator.type == :mult
+                return AssignmentCommand.new(left_hand_side, :scale, operand1.symbol, operand2.value)
+              else
+                return AssignmentCommand.new(left_hand_side, :scale, operand1.symbol, 1.0 / operand2.value)
+              end
+            elsif operand2.type == :id
+              case operator.type
+              when :plus
+                return AssignmentCommand.new(left_hand_side, :add, operand1.symbol, operand2.symbol)
+              when :minus
+                return AssignmentCommand.new(left_hand_side, :sub, operand1.symbol, operand2.symbol)
+              when :mult
+                return AssignmentCommand.new(left_hand_side, :mult, operand1.symbol, operand2.symbol)
+              end
+            end
           end
+        else
+          return AssignmentCommand.new(left_hand_side, :assign, parse_polynomial(tokenizer), nil)
         end
       elsif tokenizer.lookahead.type == :comma
         n = tokenizer.next_token