+class unknown_keyword: public runtime_error
+{
+public:
+ unknown_keyword(const std::string &k):
+ runtime_error(k)
+ { }
+
+ virtual ~unknown_keyword() throw() { }
+};
+
+
+class invalid_signature: public runtime_error
+{
+public:
+ invalid_signature(const std::string &k, const std::string & s):
+ runtime_error(format("%s %s", k, s))
+ { }
+
+ virtual ~invalid_signature() throw() { }
+};
+
+