def delegate(method, &convproc)
      convproc ||= lambda{|v,t| v.send(method) }
      upon(lambda{|v,t| v.respond_to?(method) }, convproc)
    end
def delegate(method, &convproc)
      convproc ||= lambda{|v,t| v.send(method) }
      upon(lambda{|v,t| v.respond_to?(method) }, convproc)
    end
Adds an upon rule that works by delegation if the value responds to `method`.

 Example:

     Myrrha.coercions do |r|
       r.delegate(:to_foo)

       # is a shortcut for
       r.upon(lambda{|v,_| v.respond_to?(:to_foo)}){|v,_| v.to_foo}
     end