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boost::proto::deduce_domain — A pseudo-domain for use in functions and metafunctions that require a domain parameter. It indicates that the domain of the parent node should be inferred from the domains of the child nodes.
// In header: <boost/proto/domain.hpp> struct deduce_domain { };
            When proto::deduce_domain is used as a domain — either
            explicitly or implicitly by
            proto::make_expr(),
            proto::unpack_expr(),
            or Proto's operator overloads — Proto will use the domains of the child expressions to
            compute the domain of the parent. It is done in such a way that (A) expressions in domains
            that share a common super-domain are interoperable, and (B) expressions that are in
            the default domain (or a sub-domain thereof) are interoperable with all
            expressions. The rules are as follows:
            
proto::default_domain,
                proto::basic_default_domain
                and all their sub-domains are weaker than all other domains.
              proto::basic_default_domain
                is weaker than
                proto::default_domain.
              proto::default_domain or
                proto::basic_default_domain.
              proto::default_domain or
                proto::basic_default_domain.
              proto::default_domain or
                proto::basic_default_domain
                and U contains an element that is not proto::default_domain or
                proto::basic_default_domain,
                it is an error.
              
Note: the above description sounds like it would be expensive to compute at compile time. In fact, it can all be done using C++ function overloading.