new error checking in SetDatExpr

    }

  }

  # check that the selected groups are in the Seurat object:

  if(!all(group_name %in% s_obj@meta.data[[group.by]])){

    groups_not_found <- group_name[!(group_name %in% s_obj@meta.data[[group.by]])]

    stop(

      paste0("Some groups in group_name are not found in the seurat_obj: ", paste(groups_not_found, collapse=', '))

    )

  }

  # columns to group by for cluster/celltype

  if(!is.null(group.by)){

    seurat_meta <- seurat_meta %>% subset(get(group.by) %in% group_name)