def replace_cas(key, value, cas, ttl=nil, options=nil)
      ttl ||= @options[:expires_in].to_i
      perform(:replace, key, value, ttl, cas, options)
    end
def replace_cas(key, value, cas, ttl=nil, options=nil)
      ttl ||= @options[:expires_in].to_i
      perform(:replace, key, value, ttl, cas, options)
    end
Conditionally add a key/value pair, verifying existing CAS, only if the
 key already exists on the server.  Returns the new CAS value if the
 operation succeeded, or falsy otherwise.