The writable_ concept.  
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#include <range/v3/iterator/concepts.hpp>
template<typename O, typename T>
concept ranges::writable_ =     
requires( O && o, T && t ) {  *o = (T &&) t,             *(O &&) o = (T &&) t,             
const_cast<iter_reference_t<O> 
const &&
>(*o) = (T &&) t,             
const_cast<iter_reference_t<O> 
const &&
>(*(O &&) o) = (T &&) t ; } 
 
The writable_ concept.
Definition: concepts.hpp:162