init research

dataframe/docs/StardustDocs/topics/group.md

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+[//]: # (title: group)
+
+<!---IMPORT org.jetbrains.kotlinx.dataframe.samples.api.Modify-->
+
+Group columns into [`ColumnsGroups`](DataColumn.md#columngroup). 
+
+```text
+group { columns }
+    .into(groupName) | .into { groupNameExpression }
+
+groupNameExpression = DataColumn.(DataColumn) -> String
+```
+
+**Reverse operation:** [`ungroup`](ungroup.md), [`flatten`](flatten.md)
+
+It is a special case of [`move`](move.md) operation.
+
+See [column selectors](ColumnSelectors.md) for how to select the columns for this operation.
+
+<!---FUN group-->
+
+```kotlin
+df.group { age and city }.into("info")
+
+df.group { all() }.into { it.type().toString() }.print()
+```
+
+<inline-frame src="resources/org.jetbrains.kotlinx.dataframe.samples.api.Modify.group.html" width="100%"/>
+<!---END-->