A flat_set is a container adaptor
that provides an associative container interface
that supports unique keys
(i.e., contains at most one of each key value) and
provides for fast retrieval of the keys themselves.
the time complexity of the operations
that insert or erase a single element from the set
is linear,
including the ones that take an insertion position iterator.
Descriptions are provided here only for operations on flat_set
that are not described in one of those sets of requirements or
for operations where there is additional semantic information.
The effect of calling a constructor or member function
that takes a sorted_unique_t argument
with a range that is not sorted with respect to key_comp(), or
that contains equal elements, is undefined.