Declaration `varying mat3 mat3one;` With value assignment `mat3one[0] = vec3(1., 1., 0.);` Transpiles to `mat3one.slice(0, 3) = [1., 1., 0.];` Which doesn't work. With arrays of matrices `varying mat3 mat3Arr[15];` `mat3Arr[4][0] = vec3(1., 1., 0.)` transpiles to `= [1, 1, 0]`