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diff --git a/chapter7/parens.cxx b/chapter7/parens.cxx
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+++ b/chapter7/parens.cxx
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+// FILE: parens.cxx
+// A small demonstration program for a stack.
+#include <cstdlib>     // Provides EXIT_SUCCESS
+#include <iostream>    // Provides cin, cout
+#include <stack>       // Provides stack
+#include <string>      // Provides string
+using namespace std;
+
+// PROTOTYPE for a function used by this demonstration program
+bool is_balanced(const string& expression);
+// Postcondition: A true return value indicates that the parentheses in the
+// given expression are balanced. Otherwise, the return value is false.
+
+int main( )
+{
+    string user_input;
+
+    cout << "Type a string with some parentheses:\n";
+    getline(cin, user_input);
+
+    if (is_balanced(user_input))
+        cout << "Those parentheses are balanced.\n";
+    else
+        cout << "Those parentheses are not balanced.\n";
+
+    cout << "That ends this balancing act.\n";
+    return EXIT_SUCCESS;
+}
+
+bool is_balanced(const string& expression)
+// Library facilities used: stack, string
+{
+    // Meaningful names for constants
+    const char LEFT_PARENTHESIS = '(';
+    const char RIGHT_PARENTHESIS = ')';
+
+    stack<char> store;    // Stack to store the left parentheses as they occur
+    string::size_type i;  // An index into the string
+    char next;            // The next character from the string
+    bool failed = false;  // Becomes true if a needed parenthesis is not found
+
+    for (i = 0; !failed  &&  (i < expression.length( )); ++i)
+    {
+        next = expression[i];
+        if (next == LEFT_PARENTHESIS)
+            store.push(next);
+        else if ((next == RIGHT_PARENTHESIS) && (!store.empty( )))
+            store.pop( ); // Pops the corresponding left parenthesis.
+        else if ((next == RIGHT_PARENTHESIS) && (store.empty( )))
+            failed = true;
+    }
+
+    return (store.empty( ) && !failed);
+}