The predicate_ concept.  
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#include <range/v3/functional/concepts.hpp>
template<typename Fun, typename... Args>
concept ranges::predicate_ =     
requires( Fun && fn ) {  concepts::requires_<                 convertible_to<                     decltype(invoke((Fun &&) fn, std::declval<Args>()...)),                     
bool>> ; } 
 
The predicate_ concept.
Definition: concepts.hpp:69