I would like to understand the linear approximation of $(x+y) \bmod 2^n$ by linear masking method in the paper refereed ""Improved Linear Distinguis'hers for SNOW 2.0" by Kaisa Nyberg and Johan Wall´en." In Page number 14 to 15, one theory has been given. I am unable to understand the $A_r$ matrix computation. Specially what is $v$ in this matrix. Advance thanks for anyone's kind explanation.