private boolean containsTag(String childTagName, Element parent) {
        for (Element child : XML.getChildren(parent)) {
            if (child.getTagName().equals(childTagName) || containsTag(childTagName, child))
                return true;
        }
        return false;
    }
Reads the content of an (implicit or explicit) instance tag producing an instances step

@param instanceNameString a comma-separated list of the names of the instances this is for
@param instanceElement the element having the content of this instance
@param parentTag the parent of instanceTag (or the same, if this instance is implicitly defined, which means instanceTag is the root)
@return the instances specified, one for each instance name element
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java