Issue DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE, v3

[Kim A. Barrett <IIM@ECLA.USC.EDU>]

> Additional arguments that do not correspond to slot names but are merely
> present to supply values used in subsequent initialization computations are
> allowed. 

Actually, this wasn't what I had intended when I included [svar].  I was
assuming that svar would also have to match a slot name (and that's how I
implemented it).  CLtL doesn't say anything about svars at all in DEFSTRUCT
constructors.  I think your proposal is probably more generally useful.

kab
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