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[0134] In formulae (la) and (1 b ), Mq+ is an onium cation. Onium cations having the formulae (2a ), (2b) and (2c) are preferred. 

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[0135] Herein Rq 11 to Rq 19 are each independently a C$_{1 }$-C$_{20 }$monovalent hydrocarbon group which may contain a heteroatom. Any two of Rq$^{11 }$to Rq$^{13 }$may bond together to form a ring with the sulfur atom to which they are attached. Rq$^{14 }$and Rq$^{15 }$may bond together to form a ring with the iodine atom to which they are attached. Any two ofRq$^{16 }$to Rq$^{19 }$may bond together to form a ring with the nitrogen atom to which they are attached. The monovalent hydrocar- 

