mat1 + fac2 mat2 + ...
calculates result = fac1 mat1 + fac2 mat2 + ...
The strings result, mat1, mat2 are internal names specifying the matrices. mat1, mat2 must exist, otherwise an error occurs. If result does not exist, it is created.
The factors fac1, fac2 are optional (may be variables). If not given, one is assumed.
The nuclear values associated to the individual matrices are added accordingly and the result is associated to result.