keep rows in data frame that, for all combinations of the values of certain columns, contain the same elements in another column

Question

df = pd.DataFrame({\'a\':[\'x\',\'x\',\'x\',\'x\',\'x\',\'y\',\'y\',\'y\',\'y\',\'y\'],\'b\':[\'z\',\'z\',\'z\',\'w\',\'w\',\'z\',\'z\',\'w\',\'w\',\'w\'],\'c\':[\'c