<｜begin▁of▁sentence｜>### Instruction:
As a Forensic Scientist in networkx, you are tasked with analyzing data gathered from various unique identifiers generated by the UniqueIdGenerator API. How can you utilize the `__len__` method to determine the total number of unique items stored within the generator, aiding in your investigation and analysis of these critical pieces of evidence?
### Response:
from networkx.utils import UniqueIdGenerator

# Instantiate a UniqueIdGenerator
gen = UniqueIdGenerator()

# Add some ids
gen[5]
gen[10]
gen['a']

# Get the number of unique items
count = len(gen)
print('Number of unique items:', count)
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