Processing starts with the independent variable value given by
START+BIAS and continues until a value at or beyond START+BIAS+DUR is
reached.  When interpolation mode is enabled, the STEP control
determines the value for successive processing points (called the 'step
value' or 'step time').  The number of data points processed is
DUR/STEP + 1.

When interpolation mode is disabled, processing takes place only at the
data points contained in the data file(s) being processed.  The step
time is reset at each processing point to the value taken from each
file being processed.  Multiple data files may be processed in
expressions, but they must have the same step times; otherwise the
value calculated for the expression may be meaningless.  (See the
INTERP control description.)
