How to override invoke?

[Andrew Whitworth]

The attached code works and does what you would think it should.
However, there is one snag:

$P0($P0)

This is because when you override the invoke vtable method from PIR,
you don't have access to the "self" pmc by default. You have to pass
it explicitly.

--Andrew Whitworth

.sub main :main
    $P1 = compreg 'PIR'
    $P2 = $P1(".sub a\nsay'hi from PIR'\n.end\n")
    $P2 = $P2[0]
    $P2()

    $P0 = new 'MyProc'
    assign $P0, $P2
    $P0($P0)
.end

.namespace ['MyProc']

.sub class_init :anon :init
    $P0 = get_class 'Sub'
    $P1 = subclass $P0, 'MyProc'
    addattribute $P1, 'RealSub'
.end

.sub invoke :vtable :method
    .local pmc p
    $P0 = getattribute self, 'RealSub'
    say "First"
    $P0()
    say "Last"
.end

.sub assign_pmc :vtable :method
    .param pmc p
    setattribute self, 'RealSub', p
.end