The type must be a model of . It also models Unary Polymorphic Function. The type must be a model of Expr A Domain creates an association between expressions and a so-called generator, which is a function that maps an expression in the default domain to an equivalent expression in this Domain. It also associates an expression with a grammar, to which all expressions within this Domain must conform. The grammar to which every expression in this Domain must conform. A Unary Polymorphic Function that accepts expressions in the default domain and emits expressions in this Domain. The Domain that is a super-domain of this domain, if any such domain exists. If not, it is some unspecified type. The type of the result of applying proto_generator to the specified expression type. The result is required to model Expr. The domain type associated with result_type (result_type::proto_domain) is required to be the same type as this Domain. The result of converting some type to a Proto expression type in this domain. This is used, for instance, when calculating the type of a variable to hold a Proto expression. as_expr_result_type models Expr. The result of converting some type to a Proto expression type in this domain. This is used, for instance, to compute the type of an object suitable for storage as a child in an expression tree. as_child_result_type models Expr. The result of applying proto_generator to the specified expression. The result of converting some object to a Proto expression in this domain. It returns a Proto expression object that is suitable for storage in a variable. It should return a new object, which may be a copy of the object passed in. The result of converting some object to a Proto expression in this domain. It returns an object suitable for storage as a child in an expression tree, which may simply be a reference to the object passed in.