Revert everything related to Damien's work in 2008 (he will commit a new version soon).

Here are the reverted patches:
8c0d1003b0,
25a3114287,
9afbaf6342,
dc0005f4e1,
543190f2bc.

src/ltlenv/environment.hh

@@ -23,7 +23,7 @@
 # define SPOT_LTLENV_ENVIRONMENT_HH
 
 # include "ltlast/formula.hh"
-# include "internal/environment.hh"
+# include <string>
 
 namespace spot
 {
@@ -31,7 +31,37 @@ namespace spot
   {
     /// \brief An environment that describes atomic propositions.
     /// \ingroup ltl_essential
-    typedef spot::internal::environment<ltl_t> environment;
+    class environment
+    {
+    public:
+      /// \brief Obtain the formula associated to \a prop_str
+      ///
+      /// Usually \a prop_str, is the name of an atomic proposition,
+      /// and spot::ltl::require simply returns the associated
+      /// spot::ltl::atomic_prop.
+      ///
+      /// Note this is not a \c const method.  Some environments will
+      /// "create" the atomic proposition when requested.
+      ///
+      /// We return a spot::ltl::formula instead of an
+      /// spot::ltl::atomic_prop, because this
+      /// will allow nifty tricks (e.g., we could name formulae in an
+      /// environment, and let the parser build a larger tree from
+      /// these).
+      ///
+      /// \return 0 iff \a prop_str is not part of the environment,
+      ///   or the associated spot::ltl::formula otherwise.
+      virtual formula* require(const std::string& prop_str) = 0;
+
+      /// Get the name of the environment.
+      virtual const std::string& name() = 0;
+
+      virtual
+      ~environment()
+      {
+      }
+    };
+
   }
 }