15. The system defined in claim 14 , said processing unit further comprising at least one masking matrix that is a sum of an identity matrix and at least one summand masking matrix multiplied by a weight, said summand masking matrix setting certain components of a fourth orthogonal expansion of a subvector of a fourth feature vector input to said processing unit equal to zero, as said masking matrix is multiplied to said fourth orthogonal expansion, said fourth orthgonal expansion comprising components of said subvector of said fourth feature vector and a plurality of products of said components of said subvector of said fourth feature vector.
15. The system defined in claim 14 , said processing unit further comprising at least one masking matrix that is a sum of an identity matrix and at least one summand masking matrix multiplied by a weight, said summand masking matrix setting certain components of a fourth orthogonal expansion of a subvector of a fourth feature vector input to said processing unit equal to zero, as said masking matrix is multiplied to said fourth orthogonal expansion, said fourth orthgonal expansion comprising components of said subvector of said fourth feature vector and a plurality of products of said components of said subvector of said fourth feature vector. 18. The system defined in claim 15 , wherein said weighted sum of a vector with components all equal to 1 and a label of a second feature vector input to said processing unit is a sum of said label of said second feature vector and a vector with components all equal to 1.