code: @GwtIncompatible("incompatible method")
      public static int indexOfThrowable(final Throwable throwable, final Class<?> clazz) {
          return indexOf(throwable, clazz, 0, false);
      }
docs: <p>Returns the (zero based) index of the first <code>Throwable</code>
  that matches the specified class (exactly) in the exception chain.
  Subclasses of the specified class do not match - see
  {@link #indexOfType(Throwable, Class)} for the opposite.</p>
  
  <p>A <code>null</code> throwable returns <code>-1</code>.
  A <code>null</code> type returns <code>-1</code>.
  No match in the chain returns <code>-1</code>.</p>
  
  @param throwable  the throwable to inspect, may be null
  @param clazz  the class to search for, subclasses do not match, null returns -1
  @return the index into the throwable chain, -1 if no match or null input