public static boolean isRunnableSpec(Class<?> clazz) {
    return isSpec(clazz) && !Modifier.isAbstract(clazz.getModifiers());
  }
Checks if the given class is a Spock specification (according to <tt>isSpec()</tt>),
and throws an <tt>InvalidSpecException</tt> with a detailed explanation if it is not.
spock-core/src/main/java/org/spockframework/runtime/SpecUtil.java