(6R,7R)-7-[(Z)-2-(2-Aminothiazol-4-yl)-2-methoxyiminoacetylamino]-3-[(5-methyl-2H-tetrazol-2-yl)methyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-3-ene-2-carboxylic acid pivaloyloxymethyl ester