diff --git a/lib/ruby_algebra/command.rb b/lib/ruby_algebra/command.rb
index c9f517e..3a7c643 100644
--- a/lib/ruby_algebra/command.rb
+++ b/lib/ruby_algebra/command.rb
@@ -21,6 +21,8 @@ module RubyAlgebra
       result += @lhs.is_a?(Array) ? @lhs.join(', ') : @lhs.to_s
       if @operation == :diff
         result += ' := Diff(' + @operand1.to_s + @operand2.map { |v| ", #{v}"}.join + ')'
+      elsif @operation == :subs
+        result += ' := Subs(' + @operand1.to_s + @operand2.map { |variable, value| ", #{variable}=#{value}"}.join + ')'
       else
         result += ' := ' + @operand1.to_s
         case @operation
diff --git a/lib/ruby_algebra/parser.rb b/lib/ruby_algebra/parser.rb
index 80ef029..5ff60d5 100644
--- a/lib/ruby_algebra/parser.rb
+++ b/lib/ruby_algebra/parser.rb
@@ -68,6 +68,14 @@ module RubyAlgebra
       end
     end
 
+    # Знак равенства =
+    class EqualsToken < Token
+      def initialize
+        super()
+        @type = :equals
+      end
+    end
+
     # Запятая
     class CommaToken < Token
       def initialize
@@ -141,7 +149,7 @@ 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
@@ -161,6 +169,8 @@ module RubyAlgebra
               token = ParenthesisToken.new(true)
             when ','
               token = CommaToken.new
+            when '='
+              token = EqualsToken.new
             end
             @i += 1
             if c == ':' && @expr[@i] == '='
@@ -234,7 +244,43 @@ module RubyAlgebra
             end
             return AssignmentCommand.new(left_hand_side, :diff, target, diff_variables)
           elsif operand1.symbol == 'Subs'
-            raise NotImplementedError
+            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
+            substitutions = {}
+            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
+              variable = n.symbol
+              unless tokenizer.next_token.type == :equals
+                raise StandardError, "unexpected token #{n.type}, expected ="
+              end
+              n = tokenizer.next_token
+              unless n.type == :num
+                raise StandardError, "unexpected token #{n.type}, expected number"
+              end
+              substitutions[variable] = n.value
+            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, :subs, target, substitutions)
           else
             tokenizer.next_token
             operator = tokenizer.next_token