diff --git a/projects/06/calculator08buggy.cpp b/projects/06/calculator.cpp
similarity index 82%
rename from projects/06/calculator08buggy.cpp
rename to projects/06/calculator.cpp
index da4f1e2..c0ad85b 100644
--- a/projects/06/calculator08buggy.cpp
+++ b/projects/06/calculator.cpp
@@ -1,10 +1,7 @@
-/*
-  calculator08buggy.cpp
-
-  Helpful comments removed.
-
-  We have inserted 3 bugs that the compiler will catch and 3 that it won't.
-*/
+//
+// This is example code from Chapter 7.8 "Variables" of
+// "Software - Principles and Practice using C++" by Bjarne Stroustrup
+//
 
 #include <std_lib_facilities.h>
 
@@ -17,6 +14,8 @@ struct Token
   Token(char ch) : kind{ch}, value{0} {}
 
   Token(char ch, double val) : kind{ch}, value{val} {}
+
+  Token(char ch, string id) : kind{ch}, name{id} {}
 };
 
 class Token_stream
@@ -51,6 +50,7 @@ constexpr char let = 'L';
 const string prompt = "> ";
 const string result = "= ";
 const string declkey = "let";
+const string quitkey = "quit";
 
 Token Token_stream::get()
 {
@@ -72,7 +72,7 @@ Token Token_stream::get()
   case '*':
   case '/':
   case '%':
-  case ';':
+  case print:
   case '=':
     return Token{ch};
 
@@ -100,15 +100,18 @@ Token Token_stream::get()
       string s;
       s += ch;
       while (cin.get(ch) && (isalpha(ch) || isdigit(ch)))
-        s = ch;
+        s += ch;
       cin.putback(ch);
 
       if (s == declkey)
         return Token{let};
 
+      if (s == quitkey)
+        return Token{quit};
+
       return Token{name, s};
     }
-    error("Bad token");
+    error("bad token");
   }
 }
 
@@ -121,10 +124,11 @@ void Token_stream::ignore(char c)
   }
   full = false;
 
-  char ch;
-  while (cin >> ch)
+  for (char ch; cin >> ch;)
+  {
     if (ch == c)
       return;
+  }
 }
 
 struct Variable
@@ -137,22 +141,24 @@ struct Variable
 
 vector<Variable> var_table;
 
-double get_value (string s)
+double get_value (const string& s)
 {
-  for (int i = 0; i < var_table.size(); ++i)
-    if (var_table[i].name == s)
-      return var_table[i].value;
+  for (const auto& var : var_table)
+  {
+    if (var.name == s)
+      return var.value;
+  }
 
   error("get: undefined name ", s);
 }
 
-void set_value (string s, double d)
+void set_value (const string& s, double d)
 {
-  for (int i = 0; i <= var_table.size(); ++i)
+  for (auto& var : var_table)
   {
-    if (var_table[i].name == s)
+    if (var.name == s)
     {
-      var_table[i].value = d;
+      var.value = d;
       return;
     }
   }
@@ -160,16 +166,18 @@ void set_value (string s, double d)
   error("set: undefined name ", s);
 }
 
-bool is_declared (string s)
+bool is_declared (const string& s)
 {
-  for (int i = 0; i < var_table.size(); ++i)
-    if (var_table[i].name == s)
+  for (const auto& var : var_table)
+  {
+    if (var.name == s)
       return true;
+  }
 
   return false;
 }
 
-double define_name (string var, double val)
+double define_name (const string& var, double val)
 {
   if (is_declared(var))
     error(var, " declared twice");
@@ -194,6 +202,7 @@ double primary ()
     t = ts.get();
     if (t.kind != ')')
       error("'(' expected");
+    return d;
   }
 
   case '-':
@@ -235,6 +244,15 @@ double term ()
       break;
     }
 
+    case '%':
+    {
+      double d = primary();
+      if (d == 0)
+        error("'%': divide by zero");
+      left = fmod(left, d);
+      break;
+    }
+
     default:
       ts.putback(t);
       return left;
diff --git a/projects/06/test_ans.txt b/projects/06/test_ans.txt
new file mode 100644
index 0000000..f002515
--- /dev/null
+++ b/projects/06/test_ans.txt
@@ -0,0 +1,54 @@
+> = 0
+> = 1
+> = 2
+> = 3
+> = 4
+> = 5
+> = 6
+> = 7
+> = 8
+> = 9
+> = 3
+> = 3
+> = 6
+> = -1
+> = -1
+> = -4
+> = 2
+> = 2
+> = 6
+> = 0.5
+> = 0.5
+> = 0.166667
+> = 7
+> = -5
+> = 1.66667
+> = 0.333333
+> = divide by zero
+> = primary expected
+> = 3
+> = -3
+> = 3
+> = -3
+> = 3
+> = 10
+> = 21
+> = divide by zero
+> = bad token
+> = bad token
+> = 1
+> = 21
+> = -4.5
+> = 0
+> = 0
+> = 2
+> = 0
+> = 4.2
+> = 3.3
+> = 0.2
+> = 2
+> = 3
+> = 0
+> = 5
+> = 1
+> 
\ No newline at end of file
diff --git a/projects/06/test_in.txt b/projects/06/test_in.txt
new file mode 100644
index 0000000..037dc66
--- /dev/null
+++ b/projects/06/test_in.txt
@@ -0,0 +1,50 @@
+0; 1; 2; 3; 4; 5; 6; 7; 8; 9;
+
+1 + 2;  1+2;  1 + 2 + 3;
+1 - 2;  1-2;  1 - 2 - 3;
+
+1 * 2;  1*2;  1 * 2 * 3;
+1 / 2;  1/2;  1 / 2 / 3;
+
+1 + 2*3;  1 - 2*3;
+1 + 2/3;  1 - 2/3;
+
+3/0;
+
+();
+(3);
+-(3);
++(3);
+(-3);
+(+3);
+(3+7);
+(3*7);
+
+3 / (3 - 3);
+
+3 * 4&5;
+
+93"43-189'3dfrer4734***&340#$%%%8340909909()()*(&)))((( ;
+
+4 / (2 + 2*1);
+1 + 4 * (2 + 3/2*2);
+
+72 / (10*(9-5) - 7*8);
+
++2 - 2;  -2 + 2;
+
+2 % 4;
+4 % 2;
+4.2 % 8;
+3.3 % 6.6;
+6.8 % 3.3;
+
+let x = 2;  let q=3;
+
+x*q - (x * q);
+
+q + x;
+
+x/(x + 2*q - q*2);
+
+quit