[#predicate]
[section Predicate]

A ['predicate] (or unary predicate) is a
[link metafunction_class template metafunction class] taking one argument and
returning a [link boxed_value boxed value] of type `bool`.

For example the following predicate checks if its argument is the boxed `char`
value `x`:

  struct is_x
  {
    template <class C>
    struct apply
    {
      static constexpr bool value = (C::value == 'x');
      using type = apply;
    };
  
    using type = is_x;
  };

[endsect]