@Override
    public String toString() {
        String sourceString = "";
        if ( ! argument.equals(fieldName))
            sourceString = " (argument: " + argument + ")";
        return "field " + fieldName + ": " + transform + sourceString;
    }
Returns the argument of this (used as input to the backend docsum rewriter)
config-model/src/main/java/com/yahoo/schema/derived/FieldResultTransform.java