two operants

Zwei Operanten hintereinander ist nicht erlaubt

src/calc.cpp

@@ -32,6 +32,8 @@ int evaluate(const std::string &s) {
     case '/': change_stack_state(divide,   stack); break;
     case '%': change_stack_state(modulo,   stack); break;
     case '0'...'9': stack.push(c - '0'); break;
+    default:
+      throw std::invalid_argument( "received unrecognized character" );
     }
   }