This specifies the set of one or more address towers to add to the node
 registration.

     TOWERS {tower, tower, ...}

 Each address tower in the set has the format:

     transport/address

 Fields can be omitted from left to right, and assume a default value.

     Transport can be either TP4, TP4=tsel, or NSP, and defaults to NSP
     if the N-Sel in the address field is "20", and to TP4 otherwise.

     Address can be either an NSAP value or a Phase IV address value.
     If a Phase IV address is specified, an NSAP is constructed using
     the specified address and the Phase IV prefix value.  The N-Sel
     value is always "20".

 Assuming that the Phase IV address prefix value is "49::", some examples
 of address towers are:

   Abbreviation                      Fully Specified
   ------------                      ---------------
   1.5                               NSP/49::01:AA0004000504:20
   1.5+39:840                        NSP/39:840:01:AA0004000504:20
   39:840:01:AA0004000504:20         NSP/39:840:01:AA0004000504:20
   39:840:01:AA0004000504:21         TP4=DEC0/39:840:01:AA0004000504:21
   TP4/39:840:01:AA0004000504:21     TP4=DEC0/39:840:01:AA0004000504:21

 Examples of address towers using normal default values for a DECnet
 Phase IV node and for a DECnet-Plus node might be:

   TOWERS={1.5}
   TOWERS={2.54, 39:840:01:080043A751F4:20, 39:840:01:080043A751F4:21}

 See the ADDRESS_TOWERS topic for details about the address towers.