add diff command variant

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