# 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.