Predefined Variables are variables that do not need to be created via
an Assignment statement.  That is, they are variables whose values are
pre-determined by DAN.  Their use in an Assignment statement causes the
symbol on the left of the equal sign to be a 'temporary variable'.

The following variables are predefined by DAN and may be
freely used in any expression.

T    Current step time value.

X    Current X axis value.  Normally this variable is the same
     as T except when the values in a data file are being
     used for the X axis (see XAXIS control). Be careful since
     this distinction can produce results which are not
     anticipated.
+
OPER Value to be entered by the operator via the standard
     input device for each step time. A prompt is issued
     before each data value unless this prompt is
     suppressed via the -P command line option.  Values
     entered may be floating point numbers, dates (mm/dd/yy),
     or hours (hh:mm:ss).  If a prompt is output, its format
     conforms to the current XFORMAT setting (NOMINAL,
     DATES, HOURS).

     Since the value for OPER is taken from the standard
     input device, the you may wish to suppress the prompt
     and use DOS's I/O redirection feature to input data from
     an ASCII file containing just data values (no
     time tags). This file must be terminated with a control-Z.
+
These predefined variables allow the following types of statements:

     function = I*SIN (T) + COS (J*T);
     thrust = 1000 * EXP(X);
     myfile = OPER*OPER - 2*OPER + 36.9564;

The statement creating 'myfile' causes the operator to be prompted for
a value at each step time in the implicit loop (unless the prompt has
been suppressed via the 'Suppress OPER prompt? option).  The value
entered into 'myfile' for that step time is the operator's value
squared minus two times the operator's value plus 36.9564.

In other words, the value received from the operator is used for
each occurrence of the variable OPER in the expression; only one
value is entered per step time.  An early end to OPER data can be
indicated by entering a control-Z character instead of a value;
otherwise DUR/STEP + 1 values must be entered.  If an early end
occurs, DAN completes the implicit loop and uses the last entered
value for all subsequent step times.
