What is MAY NOT

In section:

  4.2.3. Packet Length Examples
  [....]
     they literal, compressed, or encrypted. The first partial length
     MUST be at least 512 octets long. Partial Body Lengths MAY NOT be
     used for any other packet types.

I did not found MAY NOT in rfc2119 and I guess is should be replaced
by SHOULD NOT.


Werner