Creating an empty set

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}

You can just construct a set:

>>> s = set()

will do the job.