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Scattering assignment

I've gotten to really like the scattering assignment in the new server
version. I was wondering what people would think of adding on to it the
following:

  Type requirements.

So in addition to {target, ...} = expr where target now is one of:

   variable
   ?variable
   ?variable = default-expr
   @variable

One could also use other (optional) special characters besides the ?
and @ which would signify that the element of the expression corresponding 
to target must correspond to the specified data type. If not, then raise
E_TYPE. This could eliminate a lot of data checking that a large
percentage of verbs need to do.

So for each of the types:  STR, INT, FLOAT, OBJ, ERR, LIST, we would need
a special character prefix. Now, I haven't given much thought to what
characters to use for each (though # immediately comes to mind for OBJ),
and I suppose that whatever characters are chosen would have to be noted
carefully so that people don't accidently start variables with them (and
unintentionally force their data type if used in a scatter assignment),
but I think that's only a minor problem.

So, for example (using # as the prefix for type OBJ):

{x, #y, ?z=0} = {54.7, #2}       =>   x=54.7, y=#2, z=0
{x, #y, ?z=0} = {54.7, 0, "foo"}  =>   E_TYPE
{x, y, ?z=0}  = {54.7, 0, "foo"}  =>   x=54.7, y=0, z="foo"

Now, the above suggests that it might be nice to combine the ? with the
type specifier so that z must to be either STR or NUM but not
both, but I think that's probably getting a bit complex. 

Comments?

CDRS
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