what do :copiers do?

[Gregor.pa@Xerox.ARPA]

Assuming that the functionality of being able to copy a structure and
get back a structure of the same type and the functionality of being
able to copy a particular "sub-type part" of a structure are worthwhile,
here is a scheme which seems nice:

   Remember that: 
   (defstruct person name)
   (defstruct (astronaut (:include person)) helmet-size)

In the scheme I am proposing:
- copy-person of an astronaut returns a person
- a "new" function, copy, which given an astronaut returns an astronaut

More abstractly, 
COPY-xxx as defined by a defstruct returns a structure  of that
defstruct's type.

COPY returns a structure of the same type as its argument.