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Models J
flip =: 0&|: dotcj =: 2 : 'u/@:v"1/ flip' lr =: 1 : '1{u b. 0' NB. left rank dotrj =: 2 : 'u/@(v"(1+(v lr),_))' dotrj0 =: 2 : 'u/@(v"1 _)'