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

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

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

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