I want to use the spkg 'coxeter3', and I installed it successfully because it shows up under "installed_packages()".

Here's my code:

```
def MuL(C):
W = CoxeterGroup(C, implementation='coxeter3')
List=[]
for u in W:
U = (W(v) for v in W.bruhat_interval(u, W.long_element()))
next(U)
for v in U:
ldiff = v.length()-u.length()-1
if is_even(ld):
p = W.kazhdan_lusztig_polynomial(u, v)
if p[ld//2] != 0:
List += [[u,v]]
return List
```

(ref: cannot post a link, but the code is copied from David Vogan)

I run' MuL(["A",4]) ' for example:

```
<ipython-input-11-10e92b1812f5> in <module>()
----> 1 MuL(["A",Integer(4)])
<ipython-input-10-cb3068dc75a6> in MuL(C)
1 def MuL(C):
----> 2 W = CoxeterGroup(C, implementation='coxeter3')
3 List=[]
4 for u in W:
5 U = (W(v) for v in W.bruhat_interval(u, W.long_element()))
/Applications/SageMath-8.1.app/Contents/Resources/sage/local/lib/python2.7/site-packages/sage/combinat/root_system/coxeter_group.pyc in CoxeterGroup(data, implementation, base_ring, index_set)
141 from sage.libs.coxeter3.coxeter_group import CoxeterGroup
142 except ImportError:
--> 143 raise RuntimeError("coxeter3 must be installed")
144 else:
145 return CoxeterGroup(cartan_type)
RuntimeError: coxeter3 must be installed
```

what should I do to fix it?

Note:

mac os

spkg was installed not in terminal using (!sage -i coxeter3)

I used Jupiter to run the code but the same error appears using shell