@Override
    public String toString() {
        return "SpanTree '" + name + "' with root: " + root +
               ( annotations.annotations().size() > 5 ? "" :
                 ", annotations: " + annotations.annotations().stream().map(Annotation::toString).collect(Collectors.joining(", ")));
    }
Returns the StringFieldValue that this SpanTree belongs to.

@return the StringFieldValue that this SpanTree belongs to, if any, otherwise null.
document/src/main/java/com/yahoo/document/annotation/SpanTree.java