array.splitAt(x, is);
// x:  an array
// is: indices (sorted)

const array = require('extra-array');

var x = [1, 2, 2, 3, 5, 4, 4, 7];
array.splitAt(x, [1, 2, 5, 6]);
// [ [ 1 ], [ 3, 5 ], [ 7 ] ]

var x = [2, 4, 5, 6, 8];
array.splitAt(x, [0, 1, 3, 4]);
// [ [ 5 ] ]