--- a/src/symtab.cc
+++ b/src/symtab.cc
@@ -29,6 +29,7 @@
 #include "error.h"
 #include "variables.h"
 #include "utils.h"
+#include "user-prefs.h"
 #include "tree-base.h"
 #include "tree-expr.h"
 #include "tree-const.h"
@@ -621,13 +622,27 @@
 {
   if (is_read_only ())
     {
-      char *tag = "symbol";
       if (is_variable ())
-	tag = "variable";
+	{
+	  if (user_pref.read_only_constants)
+	    {
+	      if (user_pref.read_only_constants < 0)
+		{
+		  ::warning ("redefinition of constant `%s'", nm);
+		  return 0;
+		}
+	      else
+		::error ("can't redefine read-only constant `%s'", nm);
+	    }
+	}
       else if (is_function ())
-	tag = "function";
-	
-      ::error ("can't redefine read-only %s `%s'", tag, nm);
+	{
+	  ::error ("can't redefine read-only function `%s'", nm);
+	}
+      else
+	{
+	  ::error ("can't redefine read-only symbol `%s'", nm);
+	}
 
       return 1;
     }

--- a/src/user-prefs.cc
+++ b/src/user-prefs.cc
@@ -57,6 +57,7 @@
   user_pref.print_answer_id_name = 0;
   user_pref.print_empty_dimensions = 0;
   user_pref.propagate_empty_matrices = 0;
+  user_pref.read_only_constants = 1;
   user_pref.resize_on_range_error = 0;
   user_pref.return_last_computed_value = 0;
   user_pref.save_precision = 0;
@@ -345,6 +346,15 @@
   return 0;
 }
 
+// Should built-in constants always be read only?
+
+int
+read_only_constants (void)
+{
+  user_pref.read_only_constants = check_str_pref ("read_only_constants");
+
+  return 0;
+}
 
 // When doing assignments, should we resize matrices if the indices
 // are outside the current bounds?

--- a/src/user-prefs.h
+++ b/src/user-prefs.h
@@ -41,6 +41,7 @@
   int print_answer_id_name;
   int print_empty_dimensions;
   int propagate_empty_matrices;
+  int read_only_constants;
   int resize_on_range_error;
   int return_last_computed_value;
   int save_precision;
@@ -82,6 +83,7 @@
 extern int print_answer_id_name (void);
 extern int print_empty_dimensions (void);
 extern int propagate_empty_matrices (void);
+extern int read_only_constants (void);
 extern int resize_on_range_error (void);
 extern int return_last_computed_value (void);
 extern int silent_functions (void);

--- a/src/variables.cc
+++ b/src/variables.cc
@@ -1571,6 +1571,12 @@
 	  "true", 0, 0, 1, propagate_empty_matrices,
     "operations on empty matrices return an empty matrix, not an error");
 
+#if 0
+  DEFVAR ("read_only_constants", SBV_read_only_constants, "true", 0,
+        0, 1, read_only_constants,
+    "allow built-in constants to be modified");
+#endif
+
   DEFVAR ("realmax", SBV_realmax, DBL_MAX, 1, 1, 1, 0,
     "realmax (): return largest representable floating point number");