def new_context(description, klass, &definition)
      (@contexts << klass.new(description, self, &definition)).last
    end
Creates a new context instance and appends it to the set of immediate children sub-contexts.

@param [String] description a partial description
@param [Class] klass the context class that a sub-context will be generated from
@param [lambda] definition the body of the sub-context
@return [#run] something that hopefully responds to run and is context-like