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]