When split-file processing is in effect, the first
variables in the matrix system file are the split variables, followed
by ROWTYPE_, the case-identifier
variable (if VIEW=CASE and ID are used), VARNAME_, and the variables that form the matrix.
A full set of matrix materials is written for each
split-file group that is defined by the split variables.
A split variable cannot have the same name as any
other variable that is 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 procedure.