Note that IDL 2-dimensional arrays are given as Z[A,B] where A is column ref; B is row ref
x[*] means take each element of x (separately), so
x[*] = y[*] means,
x[0] = y[0], x[1] = y[1], … up to all elements of the arrays
x[0:2] is essentially does the same but on a subset of an array; eg
take elements 0 through 2 of the array (and do “whatever”)
I read also that this notation, esp on lhs, is much much faster than
conventional looping etc.