introduce formula::is_leaf()

Fixes #307.

* spot/tl/formula.hh: Here.
* tests/python/ltlparse.py: Test it.
* NEWS: Mention it.

spot/tl/formula.hh

@@ -231,6 +231,12 @@ namespace spot
         return size_;
       }
 
+      /// \see formula::is_leaf
+      bool is_leaf() const
+      {
+        return size_ == 0;
+      }
+
       /// \see formula::id
       size_t id() const
       {
@@ -1277,6 +1283,15 @@ namespace spot
       return ptr_->size();
     }
 
+    /// \brief Whether the formula is a leaf.
+    ///
+    /// Leaves are formulas without children.  They are either
+    /// constants (true, false, empty word) or atomic propositions.
+    bool is_leaf() const
+    {
+      return ptr_->is_leaf();
+    }
+
     /// \brief Return the id of a formula.
     ///
     /// Can be used as a hash number.