Value to be entered by the operator via the standard input device for
each step time during execution of the statement(s) in which an OPER
variable appears.  A prompt is issued before each data value unless
this prompt is suppressed by a 'YES' setting for the 'Suppress OPER
prompt?' Execution option.

Note: One OPER value is input for each step time, not for each use of
      the OPER variable within a statement.

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, 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.
