Multiplicity Keyword in Subset Constraint

Software Abstractions
1

Hi,

I am encountering a problem with using Multiplicity keywords within a subset constraint.

In the Software Abstractions Revised edition, on bottom page 275 it says

In either a declaration

decl ::= [disj] name,+ : [disj] expr
or a subset constraint

expr ::= expr in expr

the right-hand side expression may include multiplicities, and the keyword set that represents the omission of a multiplicity constraint.

and on top page 290 it says

As explained in section B.7.3, a boolean expression formed with the in keyword may, like a declaration, use multiplicity symbols to impose an additional constraint.

I think the above means that I can write something like this:

fact {
x in lone S
}

But this is recognized as a syntax error in the Alloy Analyzer.

It would be great if someone could explain the semantics of such a constraint too.

Thank you,

Jack