code: public void assertTopmostElement(By by) {
  		if (!isTopmostElementCheckApplicable(by)) {
  			LOGGER.warn("The element identified by '{}' is not a leaf node in the "
  					+ "document tree. Thus, it cannot be checked if the element is topmost. "
  					+ "The topmost element check cannot be performed and is skipped.", by);
  			return;
  		}
  		LOGGER.info("Checking whether the element identified by '{}' is the topmost element.", by);
  		WebElement topmostElement = findTopmostElement(by);
  		WebElement element = findElement(by);
  		if (!element.equals(topmostElement)) {
  			throw new AssertionError(format("The element '%s' identified by '%s' is covered by '%s'.",
  					outerHtmlPreview(element), by, outerHtmlPreview(topmostElement)));
  		}
  	}
docs: Asserts that the element is not covered by any other element.
  
  @param by
  the {@link By} used to locate the element.
  @throws WebElementException
  if the element cannot be located or moved into the viewport.
  @throws AssertionError
  if the element is covered by some other element.