In DAN's language, variables are files containing multiple pairs of
floating point values.  DAN assumes that the pairs in the file
represent points (i.e.  (x,f(x)) values) on a continuous function whose
other points can be found by a linear interpolation between the given
points unless interpolation mode is disabled (see INTERP control). 
This interpolation is performed automatically by DAN at each step time
whenever it accesses a variable.  Additionally, DAN assumes that the
first value in the file is the value the function would have at any
point preceding the first point, and the last value in the file is used
for all points after the last point.

Variables are accessed by including the file name in an expression; for
example,
       PLOT ( A*B );
where A and B are variables representing the functions defined by the
data points in files A and B.
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Since DAN performs a linear interpolation to determine the value of the
function at points other than those explicitly given in the file, it is
not necessary to have exact agreement between the independent variable
values (i.e.  the x value in an x,f(x) pair) in the file and the step
times; nor between the independent variable values in different files. 
It is also not necessary to have the same number of data points in a
file as the number of step times in the processing period.
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There are four types of variables:

Temporary
     Data files created by DAN when an assignment statement is executed.
     These files are deleted after an algorithm has finished executing.

Pre-existing
     Data files preserved via the SAVE statement during a previous
     DAN invocation or created by a user's application program.  These
     files can be used directly in an expression.  They are not deleted
     after the algorithm is done.

Predefined
     Variables T, X, and OPER.

User defined
     Data consisting of time tag and data value pairs given in an
     ASCII text file that you have created or that was created by
     DAN's WRITE procedure.
