FieldPathIteratorHandler getIteratorHandler(Document doc) {
        if (expression != null) {
            try {
                return new MathAssignIteratorHandler(expression, doc, removeIfZero, createMissingPath);
            } catch (ParseException e) {
                return null;
            }
        } else {
            return new SimpleAssignIteratorHandler(fieldValue, removeIfZero, createMissingPath);
        }
    }
@return Returns true if this assignment is an arithmetic operation.
document/src/main/java/com/yahoo/document/fieldpathupdate/AssignFieldPathUpdate.java