Support for Contexts

[jeme]

In some cases it would be desired to enter a context when writing validation rules, so that we do not repeat our self. e.g.

   When(Any).Then(
                Field("person.name", !Must.Be.NullOrEmpty()) &
                Field("person.familyName", !Must.Be.NullOrEmpty()) &
                Field("person.age", !Must.Be.NullOrEmpty()) &
                Field("person.gender", !Must.Be.NullOrEmpty()) 
                );

Would be replacable by something like: (Syntax not decided)

  using(With("person")) { 
     When(Any).Then(
                Field("name", !Must.Be.NullOrEmpty()) &
                Field("familyName", !Must.Be.NullOrEmpty()) &
                Field("age", !Must.Be.NullOrEmpty()) &
                Field("gender", !Must.Be.NullOrEmpty()) 
                );
  }

To say that each of the following rules are children of the parent "person".