[Python] Creating an empty set [duplicate]

I have some code which tots up a set of selected values. I would like to define an empty set and add to it, but {} keeps turning into a dictionary. I have found if I populate the set with a dummy value I can use it, but it's not very elegant. Can someone tell me the proper way to do this? Thanks.

inversIndex = {'five': {1}, 'ten': {2}, 'twenty': {3},
'two': {0, 1, 2}, 'eight': {2}, 'four': {1},
'six': {1}, 'seven': {1}, 'three': {0, 2},
'nine': {2}, 'twelve': {2}, 'zero': {0, 1, 3},
'eleven': {2}, 'one': {0}}

query = ['four', 'two', 'three']

def orSearch(inverseIndex, query):
b = [ inverseIndex[c] for c in query ]
x = {'dummy'}
for y in b:
{ x.add(z) for z in y }
x.remove('dummy')
return x

orSearch(inverseIndex, query)

{0, 1, 2}
​

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