public SpanTree getSpanTree(String name) {
        if (spanTrees == null) {
            return null;
        }
        return spanTrees.get(name);
    }
Returns the span tree associated with the given name, or null if this does not exist.

@param name the name of the span tree to return
@return the span tree associated with the given name, or null if this does not exist.
document/src/main/java/com/yahoo/document/datatypes/StringFieldValue.java