Not Unique SOAP element

[zbaibg]

https://singroup.github.io/dscribe/latest/tutorials/descriptors/soap.html
in this page, it says the SOAP features only keep unique elements considering of symmetries.

But in the following pseudo-code (from the above page), some repetitive elements seems to be kept. e.g. when there are 2 species, n_max=2 and l_max=0, this will output 10 elements instead of 9.

for Z in atomic numbers in increasing order:
for Z' in atomic numbers in increasing order:
for l in range(l_max+1):
for n in range(n_max):
for n' in range(n_max):
if (n', Z') >= (n, Z):
append p(\chi)^{Z Z'}_{n n' l}` to output

This can also be checked from the following code, which output 10 instead of 9.

soap=SOAP(
species=['C','H'],
periodic=False,
r_cut=5.0,
n_max=2,
l_max=0,
sigma=0.5,)
soap.get_number_of_features()

The reason is from tuple comparision: (n', Z') >= (n, Z) in python. When n' is larger than n, it would not compare Z' and Z. So n,n',Z,Z' of (0,1,1,0) and (0,1,0,1) are both outputed, which I think is repetitive.