 => new Array(choices).fill(null).map(rand_string);
            const colname = (name, x) => `${name.charAt(0).toUpperCase() + name.slice(1)} ${x}`;

            function* col_iter() {
                for (const name of ["float", "integer", "string", "datetime", "boolean"]) {
                    const ncols = window[`num_${name}`].value;
                    for (let x = 0; x < ncols; x++) {
                        yield [name, x];
                    }
                }
            }

            const assi