<｜begin▁of▁sentence｜>### Instruction:
Given a MultiDict object with keys 'a' and 'b', where key 'a' is associated with multiple values, use the __iter__ method to iterate over the keys of the multidict. Note that each unique key should be printed only once, regardless of how many values it has.
### Response:
from multidict import MultiDict

# Create a multidict object with some key-value pairs
d = MultiDict({'a': 1, 'b': 2, 'a': 3})

# Using __iter__ to iterate over the keys of the multidict
for key in d.__iter__():
    print(key)

# This will output:
# a
# b
# Note that 'a' is printed once even though it appears twice in the multidict with different values.
<|EOT|>
