sig Element{}
sig Bag{ // multiset, absence treated as zero occurences
  content : Element ->one Nat
}

sig Pair{
  first, second : Element
}

Jeremy L. Jacob        Tel: +44-1904-43-2747    Fax: +44-1904-43-2767
Jeremy.Jacob@cs.york.ac.uk    http://www-users.cs.york.ac.uk/~jeremy/
Dept. of Computer Science, The University of York, YORK, YO10 5DD, UK

Thank you for your answer.

In your way, cardinality (number of member) of tuple is constant. But I need the tuple with variable cardinality. It may be 2-tuple in first time and after a while is 4-tuple.
Please, help me again.
Thanks a lot.

That is what the seq constructor is for:

sig A{
  field : seq B
}
sig B{}

Jeremy L. Jacob        Tel: +44-1904-43-2747    Fax: +44-1904-43-2767
Jeremy.Jacob@cs.york.ac.uk    http://www-users.cs.york.ac.uk/~jeremy/
Dept. of Computer Science, The University of York, YORK, YO10 5DD, UK

Another possibility (with different scope and scalability behaviors)
might be to explicitly list out the different arities:

abstract sig Tuple { }

sig Pair extends Tuple { a, b: X }
sig Tripple extends Tuple { a, b, c: X }
sig Quad extends Tuple { a, b, c, d: X }

sig Something { someField: set Tuple }

(My example lists up to arity 4.  I omit arity 5 and above,
because with the other approach, Alloy Analyzer typically
won't perform well with arity 5 or above anyway...)

seq constructor solves my problem.

I thought all things about alloy can be found in tutorial slide in address:
http://alloy.mit.edu/alloy4/tutorial/

But, there is not any things about seq in tutorial slide. Although I found it in what's new section:
http://alloy.mit.edu/alloy4/kb/seq.html

I suggest seq and another features which probably is not in tutorial slide, should be added to it.

Thanks a lot.