12.11.2 AddressList Objects 
An AddressList instance has the following methods:
- __len__()
 - 
Return the number of addresses in the address list.
 
- __str__()
 - 
Return a canonicalized string representation of the address list.
Addresses are rendered in "name" <host@domain> form, comma-separated.
 
- __add__(alist)
 - 
Return a new AddressList instance that contains all addresses
in both AddressList operands, with duplicates removed (set
union).
 
- __iadd__(alist)
 - 
In-place version of __add__(); turns this AddressList
instance into the union of itself and the right-hand instance,
alist.
 
- __sub__(alist)
 - 
Return a new AddressList instance that contains every address
in the left-hand AddressList operand that is not present in
the right-hand address operand (set difference).
 
- __isub__(alist)
 - 
In-place version of __sub__(), removing addresses in this
list which are also in alist.
 
Finally, AddressList instances have one public instance variable:
- addresslist
 - 
A list of tuple string pairs, one per address.  In each member, the
first is the canonicalized name part, the second is the
actual route-address ("@"-separated username-host.domain
pair).
 
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