When split-file processing is in effect, the first
variables in the matrix data file are the split variables, followed
by ROWTYPE_, VARNAME_ (or FACTOR_), and then the variables used to form the matrix.
A full set of matrix materials is written for each
split-file group defined by the split variables.
A split variable cannot have the same variable name
as any other variable written to the matrix data file.
If split-file processing is in effect when a matrix
is written, the same split file must be in effect when that matrix
is read by any other procedure.