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.