duckdb<- asNamespace("duckdb")drv<-duckdb::duckdb()con<-DBI::dbConnect(drv)experimental<-FALSEdf1<-tibble::tibble(a=1)rel1<-duckdb$rel_from_df(con,df1,experimental=experimental)rel2<-duckdb$rel_project(rel1,list(    {tmp_expr<-duckdb$expr_reference("a")duckdb$expr_set_alias(tmp_expr,"a")tmp_expr    },    {tmp_expr<-if ("experimental"%in% names(formals(duckdb$expr_constant))) {duckdb$expr_constant(NA,experimental=experimental)      }else {duckdb$expr_constant(NA)      }duckdb$expr_set_alias(tmp_expr,"b")tmp_expr    }  ))rel2#> DuckDB Relation:#> ---------------------#> --- Relation Tree ---#> ---------------------#> Projection [a as a, NULL as b]#>   r_dataframe_scan(0x11e727dd0)#>#> ---------------------#> -- Result Columns  --#> ---------------------#> - a (DOUBLE)#> - b (INTEGER)dput(duckdb$rel_to_altrep(rel2))#> structure(list(a = 1, b = NA_integer_), row.names = c(NA, -1L#> ), class = "data.frame")DBI::dbDisconnect(con)