Memcache.prepend_cas

You're seeing just the function prepend_cas, go back to Memcache module for more information.
Link to this function

prepend_cas(server, key, value, cas, opts \\ [])

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Specs

prepend_cas(GenServer.server(), binary(), value(), integer(), Keyword.t()) ::
  store_result()

Prepends the value to the start of the current value of the key if the CAS value is equal to the provided value

Accepted options: :cas