--- a/src/pt-cmd.h
+++ b/src/pt-cmd.h
@@ -63,7 +63,7 @@
 {
 public:
 
-  tree_no_op_command (const string& cmd = "no_op", int l = -1, int c = -1)
+  tree_no_op_command (const std::string& cmd = "no_op", int l = -1, int c = -1)
     : tree_command (l, c), orig_cmd (cmd) { }
 
   ~tree_no_op_command (void) { }
@@ -72,11 +72,11 @@
 
   void accept (tree_walker& tw);
 
-  string original_command (void) { return orig_cmd; }
+  std::string original_command (void) { return orig_cmd; }
 
 private:
 
-  string orig_cmd;
+  std::string orig_cmd;
 
   // No copying!