AMPL > >Faqs  Sets and Indexing  Why doesn’t AMPL interpret the
expression T..1 as I expect?  In general, AMPL interprets a..b
as {a, a+1, a+2, ..., b-1, b}. Thus when T is greater than one,
T..1 is an empty set. To specify the ordered set that goes from
T down to 1, write T..1 by -1.  Why does “set S := 50 .. 70″
give me a set of only 3 members?  You have declared set S in
your model and set S := 50 .. 70 in your data. Set expressions
are not recognized in AMPL’s data mode, however. Instead AMPL
tries to recognize 50 .. 70 as a space-delimited list of members
of S, with the result that it has found three members: the
numbers 50 and 70, and the string "..".  To define S to equal 50
.. 70 by use of your data file, first declare S in your model by
param begin; param end > begin; set S := begin .. end;  Then
state in the data file:  param begin := 50; param end   := 70;
Alternatively, it’s legal to give set S = 50 .. 70 as your
declaration of S. This is a less desirable approach, however,
because it moves some