Synopsis

Description

The Generator determines how new expressions in the domain are constructed.  Typically, a generator
wraps all new expressions in a wrapper that imparts domain-specific behaviors to expressions within
its domain. ( See proto::extends<>.) 

The Grammar determines whether a given expression is valid within the domain, and automatically
disables any operator overloads which would cause an invalid expression to be created.  By default,
the Grammar parameter defaults to the wildcard, proto::_, which makes all expressions valid within the domain.