Implement

lib/chervil/core.rb

       end
     end
 
+    def self.arity_check(args, expected_count)
+      if args.size != expected_count
+        ::Chervil::Error.new("Expected #{expected_count} arguments but received #{args.size}")
+      else
+        nil
+      end
+    end
+
     CORE = {
       "+" => Proc.new { |args| type_check(args, Float) || args.inject(:+) },
       "-" => Proc.new { |args| type_check(args, Float) || args.inject(:-) },
       ">=" => Proc.new { |args| type_check(args, Float) || compare_pairs(args, :>=) },
       "and" => Proc.new { |args| !(args.include?(false)) },
       "or" => Proc.new { |args| args.any? { |arg| !!arg == true } },
+      "not" => Proc.new { |args| arity_check(args, 1) || args.first == false ? true : false },
     }
   end
 end