When split-file processing is in effect, the first
variables in the matrix data file are the split variables, followed
by ROWTYPE_, the independent variable, VARNAME_, and the dependent variables.
A full set of matrix materials is written for each
split-file group defined by the split variable(s).
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 procedure.
Generally, matrix rows, independent variables, and
dependent variables can be in any order in the matrix data file read
by keyword IN. However, all split-file
variables must precede variable ROWTYPE_, and all split-group rows must be consecutive. ONEWAY ignores unrecognized ROWTYPE_ values.