Array Index Range Assignment
As per the Ion specification:
Array Index Range Assignment
It is also possible to assign an array to replace a range of values. The array being assigned to that region may be smaller or larger than the region it is replacing.
$ let FOO = [1 2 3 4]
$ let FOO[..1] = [4 5 6]
$ let FOO[2..3] = [7]
$ echo @FOO
4 5 6 7