def get_cas(key)
      (value, cas) = perform(:cas, key)
      value = (!value || value == 'Not found') ? nil : value
      if block_given?
        yield value, cas
      else
        [value, cas]
      end
    end
def get_cas(key)
      (value, cas) = perform(:cas, key)
      value = (!value || value == 'Not found') ? nil : value
      if block_given?
        yield value, cas
      else
        [value, cas]
      end
    end
Get the value and CAS ID associated with the key.  If a block is provided,
 value and CAS will be passed to the block.