throw new XRRuntimeException("Could not find sibling to: " + which);
return null;
    public Content getPreviousInFlowSibling(LayoutContext c, Content which) {        List children = getChildContent(c);        for (int i = 0; i < children.size(); i++) {            Object obj = children.get(i);            if (obj == which) {                for (int j = i - 1; j >= 0; j--) {                    Object result = children.get(j);                    if (ContentUtil.isNotInFlow(result)) {                        continue;                    }                    return (Content) result;                }            }        }        throw new XRRuntimeException("Could not find sibling to: " + which);    }