Has the form:

p[f]

where f is a function with the signatures:

void f(Attrib const&);
void f(Attrib const&, Context&);
void f(Attrib const&, Context&, bool&);

You can use Boost.Bind to bind member functions. For function objects, the allowed signatures are:

void operator()(Attrib const&, unused_type, unused_type) const;
void operator()(Attrib const&, Context&, unused_type) const;
void operator()(Attrib const&, Context&, bool&) const;

The unused_type is used in the signatures above to signify 'don't care'.

For more detailed information about semantic actions see: here.