Mesoionic Dimethyl Derivatives of Some 1,2,4-Triazinones. The Course of the Reaction of 3,5-Dimethoxy, 3-Methoxy-5-methylthio, 5-Methoxy-3- methylthio and 3,5-Bis(methylthio)-1,2,4-triazine with Methyl Iodide.