Exchanges two values.
This is part of package extra-array.
array.swap$(x, i, j);
// x: an array (updated)
// i: an index
// j: another index
// → x
const array = require("extra-array");
var x = [1, 2, 3, 4];
array.swap$(x, 0, 1);
// [ 2, 1, 3, 4 ]
x;
// [ 2, 1, 3, 4 ]
var x = [1, 2, 3, 4];
array.swap$(x, 0, 3);
// [ 4, 2, 3, 1 ]