drop(columnName) {
        return this.__newInstance__(
            this[__rows__].map((row) => row.delete(columnName)),
            this[__columns__].filter((column) => column !== columnName)
        );
    }
Remove a single column.
@param {String} columnName The column to drop.
@returns {DataFrame} A new DataFrame without the dropped column.
@example
df.drop('column2')