The command processor keeps track of a current focus value.
This value is normally the last value returned by a command.
If a command returns multiple values the focus object is a list of the
values.
The focus value is not changed if a command returns no values or
a distinguished `unspecific' value.
Examples of forms that return this unspecific value are definitions,
uses of set!, and (if #f 0).
It prints as #{Unspecific}.
The reader used by the command processor reads ## as a special
expression that evaluates to the current focus object.
> (list 'a 'b)
'(a b)
> (car ##)
'a
> (symbol->string ##)
"a"
> (if #f 0)
#{Unspecific}
> ##
"a"
>
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