Basic type attribute reference.
 Reference to an attribute in an object.

 The reference contains
   - objid of the referenced object
   - classid of body
   - offset of the attribute
   - size of the attribute
   - a flagword. If the bit Indirect is set, the attribute is a pointer.

 The string notation for an attrref is
 _A:(:).[index], e.g.
 _A0.123.34.63:1234567890(pwrb:Ai).FilterAttribute[2]

 or if the class is unknown, the notation is
 _A:().()., e.g.
 _A0.123.34.63.1234567890(_C0.1:34).(_B1)60.34

 where
   vid is the volume id for the object.
   oix is the object index for the object.
   cid is classid for the class of the object.
   bix is body index for the body of the attribute.