Homogeneous square matrix (3x3, dedicated to 2D computations), whose coordinates are floating-point values (Real). Indices ranges for 0 to 2, the first index is the abscissa (left to right index) and the second one the ordinate (top to bottom index).
[1][0] refers to the matrix element located in the second column (1) at its top (0).
For 2D needs, an HomogeneousMatrix3 [ r, t; 0, 1 ] could be used, where r is a 2x2 rotation matrix, and t a 2D translation vector.
This would however prevent from considering that an homogeneous matrix is a (particular case of) matrix (all of the matrix elements would be inherited), or force to add more complexity to the inheritance tree.