Dual iterators in constructors

Most aggregate types support various constructors (as described in their specific sections), some of which use the syntax x in S to iterate over the elements of a structure \(S\) during the construction. If this structure \(S\) supports dual iteration, then the syntax i -> x in S is also allowed in such situations (and consequently both \(i\) and \(x\) may be used in the associated expressions). The meanings assigned to \(i\) and \(x\) during dual iteration will be explained in the sections for those types which support it.