fn key_column(
        &self,
        rconfig: &Config,
        headers: &csv::ByteRecord,
    ) -> CliResult<usize> {
        let select_cols = rconfig.selection(headers)?;
        if select_cols.len() == 1 {
            Ok(select_cols[0])
        } else {
            fail!("can only partition on one column")
        }
    }
Get the column to use as a key.
src/cmd/partition.rs