> ## Documentation Index
> Fetch the complete documentation index at: https://docs.abbyy.com/llms.txt
> Use this file to discover all available pages before exploring further.

# The Optional property of a Group element

> Find out when to mark a Group element as Optional in FlexiLayout Studio, how null hypotheses behave, and why subelement calls need an IsNull check first.

When a Group element is created, the **Optional element** box is clear by default, that is, the element is required. During FlexiLayout matching, a non-null hypothesis is generated for a required Group element, even if null hypotheses are generated for all of its subelements.

To check the properties of the subelements of such a hypothesis, call the code in their **Advanced** sections. If the Group element is optional, a null hypothesis can be generated for it (if there are no non-null hypotheses with the quality higher than the quality of a null hypothesis).

## Why calls to an undetected optional group fail

Avoid selecting the **Optional element** box for Group elements.

The reason is that if the Group element is marked as **Optional element** and is not detected (its quality is lower than the quality of a null hypothesis, or the `Dontfind()` function is called for it), any call to any of its subelements will cause errors. This is because in a group with a null hypothesis, the hypotheses of subelements are not formed.

To avoid this error, you must first check the Group element. If the `IsNull` check returns `True`, do not address any of the subelements.

## When an optional Group element is useful

An optional Group element and its null hypothesis are needed when the entire group of fields is not present on the image, so searching for them serves no purpose. You can speed up the search for the elements by calling the `DontFind()` method for the Group element.

<Note>
  Here, "call" means writing code in any of the sections on the **Advanced** tab or in the block properties in the **Expression** window. If the properties of a Group element are called when setting the search constraints in **Relations**, the `IsNull` check is automatic. You can see this if you click **Code** on the **Advanced** tab.
</Note>

## The GroupSample.fsp sample project

This is illustrated in the `GroupSample.fsp` project (folder `%public%\ABBYY\FlexiCapture\12.0\Samples\FLS\Tips and Tricks\Optional Group`).

In the **Properties** dialog box of the Group element **InvoiceRequisiteGroup**, the **Optional element** box is selected.

On the **Advanced** tab, the **Advanced pre-search relations** section contains the following code:

```text theme={null}
if FormID.IsNull then Dontfind();
```

This code checks for the presence of the element which identifies the document type. The **FormID** element, created before the Group element **InvoiceRequisiteGroup**, searches for the static text with a known value ("ID2015").

If the value of the identifier on the document matches the value specified in the **Search text** section, a non-null hypothesis is generated for the **FormID** element. Otherwise, the static text **FormID** is not detected, and FlexiLayout Studio is instructed not to look for the Group element **InvoiceRequisiteGroup**. In that case, a null hypothesis is created for the optional Group element **InvoiceRequisiteGroup**.

The project contains an element **TotalSumHeader** that is used to look for the name of the sum field.

The following code is entered for the element in the **Advanced pre-search relations** section:

```text theme={null}
Below: SearchElements.InvoiceRequisiteGroup.InvoiceDateHeader, 0 * dot;
```

This code means that the search for the name will be run below the date field described by the **InvoiceDateHeader** element, which, in turn, is a subelement of the Group element **InvoiceRequisiteGroup**.

## Reproduce and fix the undefined hypothesis error

Run the FlexiLayout matching procedure on both pages in the batch. For page 1 the procedure is successful, but when the FlexiLayout is applied to page 2, FlexiLayout Studio displays the following error message:

```text theme={null}
"Page 2: Error in element "SearchElements.TotalSumHeader", Advanced generator parameters section: Attempt to access undefined hypothesis: SearchElements.InvoiceRequisiteGroup"
```

This happens because on page 2 the value of the document identifier is ID 2589. Since this value is different from the one specified in the properties of the **FormID** element, the `Dontfind()` function generated a null hypothesis for the Group element **InvoiceRequisiteGroup**. As a result, the code called a non-existent hypothesis.

The correct code must look like this.

```text theme={null}
if not( SearchElements.InvoiceRequisiteGroup.IsNull ) then
{ Below: SearchElements.InvoiceRequisiteGroup.InvoiceDateHeader, 0 * dot;}
```

<Note>
  If you comment out the code in the **Advanced pre-search relations** section and select the check box next to the similar search constraint of the **TotalSumHeader** element in the **Relations** section, clicking **Code** on the **Advanced** tab will show that the compiled code already contains the `IsNull` check.
</Note>