How to define new materials in MPM simulation

[AyumuYasue0421]

#Final goal
Successful concrete flow simulation

#Current issues
I don't know how to set a new material (e.g. concrete)
##Steps procedure
(1)Create "genesis/engine/materials/MPM/concrete.py"
(2)Modify "genesis/engine/materials/MPM/init.py"
[Modifications content]
Add "from .concrete import Concrete"
(3)Modify "examples/mpm_liquid.py"
[Modifications content]
"material=gs.materials.MPM.Liquid(),"
→"material=gs.materials.MPM.Concrete(),"

I added the concrete material code as above and applied it to the sample code. However, the following error occurred.

##Error content
"Traceback (most recent call last):
File "/home/iga/Genesis_ws/Genesis/examples/mpm_liquid_attempt.py", line 44, in
material=gs.materials.MPM.Concrete(),
AttributeError: module 'genesis.engine.materials.MPM' has no attribute 'Concrete'"

#Fundamental problem
In addition to the above problem, if I delete
"from .liquid import Liquid"
in
"genesis/engine/materials/MPM/init.py"
, the original
"examples/mpm_liquid.py"
before modification runs without any problems.

In other words, I don't seem to understand the fundamental connection between the material definition and the sample code in Genesis' MPM simulation (examples/mpm_liquid.py etc...).

#Discussion content
Please tell me how to set up a new material in the genesis/engine/materials/MPM/
folder and apply it to the MPM simulation.