def replace_cas(key, value, cas, ttl = nil, req_options = nil)
      perform(:replace, key, value, ttl_or_default(ttl), cas, req_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.