Questions regarding s/// and subst?

[Stevan Little]

Hello All,
I have been asking Autrijus about getting the substitution regexp 
working in Pugs, and a few issues have come up which need some 
clarification.

In the Smart Matching table in Synopsis 04, there is a mention of 
'subst' (here is that snippet so you don't have to go digging):

    $_      $x        Type of Match Implied    Matching Code
    ======  =====     =====================    =============
    Any     subst     substitution match*      match if $_ ~~ subst
This seems to be the first mention of 'subst', and no real 
clarification followed after. I search the other Synopsis and found it 
again, all the way a the very very very bottom of Synopsis 05, where it 
is mentioned as a method form of s/// (again here is the snippet):

        There are also method forms of C<m//> and C<s//>:
        $str.match(//);
        $str.subst(//, "replacement")
        $str.subst(//, {"replacement"})
        $str.=subst(//, "replacement")
        $str.=subst(//, {"replacement"})
This though, only seemed to muddy the waters more.
Further discussion on #perl6 lead to more questions regarding subst and 
s///. What follows is the list of questions raised by this 
conversation.

Is subst an object/type?
Or is it a method of the Str object?
If it is an object ...
        
        Does s/// produce a subst object?
        Does a subst object .hasa rule object(s)?
                If so can we extract those rule object(s) from it?
        What exactly does a subst object do?
        How does a subst object stringify?
        Etc. etc. etc.
If it is a method ...
        
        How would a method on the right hand side of ~~ work exactly?
        Is
                ~~ s:perl5:g/a/b/
        the same as
                ~~ .subst(rx:perl5/a/, "b")
        or
                ~~ .subst(rx:perl5/a/, {"b"})
Thanks,
Stevan