Counts occurrences of values.
Alternatives: one, one-map, all-map.
This is part of package extra-array.
array;// x: an array// fn: map function (v, i, x)// ths: this argument// --> occurrences
const array = ; var x = 1 1 2 2 4;array;// 2 ^ ^ var x = 1 1 2 2 4;array;// 2 ^ ^