Refers to the automatic sequencing from START to START+DUR in equal
increments given by STEP with expression evaluation done at each of
these values when DAN executes a statement that references a variable. 
Thus, the statement PLOT( exprA - exprB) means determine the values of
'exprA' and 'exprB' at START, subtract the values, and plot the result;
determine the values of 'exprA' and 'exprB' at START + STEP, subtract
the values, and plot the result; ...; determine the values of 'exprA'
and 'exprB' at START + DUR + ?, subtract the values and plot the
result.

Any statement that references a data file or one of
DAN's pre-defined variables (i.e.  T, X, OPER) causes an implicit loop
to be executed with processing taking place at each step time.
