in src/types/column-helper.ts [40:50]
private static internalCreateColumnGroup(columnDef: ColumnDef, parent?: Column): Column {
const group = new Column(columnDef, parent);
group.columns = [];
group.colSpan = 0;
for (const childColumnDef of columnDef.columns) {
const childColumn = this.createColumn(childColumnDef, group);
group.columns.push(childColumn);
group.colSpan += childColumn.colSpan;
}
return group;
}