@Override
    public StringFieldValue clone() {
        StringFieldValue strfval = (StringFieldValue) super.clone();
        if (spanTrees != null) {
            strfval.spanTrees = new HashMap<>(spanTrees.size());
            for (Map.Entry<String, SpanTree> entry : spanTrees.entrySet()) {
                strfval.spanTrees.put(entry.getKey(), new SpanTree(entry.getValue()));
            }
        }
        return strfval;
    }
Clones this StringFieldValue and its span trees.

@return a new deep-copied StringFieldValue
document/src/main/java/com/yahoo/document/datatypes/StringFieldValue.java