ks_cobuchi: forbid n=0

* spot/gen/automata.hh: State the precondition.
* spot/gen/automata.cc: Catch n==0.
* tests/core/genaut.test: Test it.

spot/gen/automata.cc

@@ -29,6 +29,8 @@ namespace spot
     twa_graph_ptr
     ks_cobuchi(unsigned n)
     {
+      if (n == 0)
+        throw std::runtime_error("ks_cobuchi() expects a positive argument");
       // the alphabet has four letters:
       // i, s (for sigma), p (for pi), h (for hash)
       // we encode this four letters alphabet thanks to two AP a and b
@@ -86,4 +88,3 @@ namespace spot
     }
   }
 }
-

spot/gen/automata.hh

@@ -41,8 +41,9 @@ namespace spot
     ///     year={2015},
     ///     organization={Springer}
     /// }
+    ///
+    /// \pre n>0
     SPOT_API twa_graph_ptr
     ks_cobuchi(unsigned n);
   }
 }
-

tests/core/genaut.test

@@ -36,3 +36,6 @@ cat >expected <<EOF
 7,20,48,1,Fin(0)
 EOF
 diff out expected
+
+genaut --ks-cobuchi=0 2>err && exit 1
+grep positive err