[Tutor] Python Error

[Benjamin Lee]

Hello,
I am using python to calculate distances across the cell. 
I used this command:  python calculate_distances.py
This was the response:
Traceback (most recent call last):
file "calculate_distances.py" line 4, in <module>
import celltool.simple_interface as si
ImportError: No module named celltool.simple_ interface


I am not sure how to correct this error.
Thank you for your time,
Ben
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