public boolean isSpanNodeValid() {
        return spanNode != null && spanNode.isValid();
    }
Returns true iff this Annotation is associated with a SpanNode and the SpanNode is valid.

@return true iff this Annotation is associated with a SpanNode and the SpanNode is valid.
@see com.yahoo.document.annotation.SpanNode#isValid()
document/src/main/java/com/yahoo/document/annotation/Annotation.java