function [A, B, C, D] = abcd2ABCD( abcd_matrix )
% decompose  abcd matrix to its coefficients
%          | A   B |
%   abcd = |       |
%          | C   D |

A = abcd_matrix(1,1);
B = abcd_matrix(1,2);
C = abcd_matrix(2,1);
D = abcd_matrix(2,2);

end