6-((3-(2,6-dichlorophenyl)-5-methyl-4-isoxazolyl)carbonyl)amino)-3,3-dimethyl-7-oxo-4-thia-1-azabi cyclo(3.2.0)heptane-2-carboxylic acid, mixt. with 6-(2,2-dimethyl-5-oxo-4-phenyl-1-imidazolidinyl)-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo(3.2.0) heptane-2-carboxylic acid