@Override
    public int hashCode() {
        int result = super.hashCode();
        result = 31 * result + (children() != null ? children().hashCode() : 0);
        return result;
    }
Returns the text that is covered by this SpanNode.

@param text the input text
@return the text that is covered by this SpanNode.
document/src/main/java/com/yahoo/document/annotation/SpanList.java