=begin pod

=TITLE class Parcel

    class Parcel is Cool does Positional { }

I<Parcel> stands for I<Par>enthesis I<cel>l, ie an expression
surrounded by parenthesis. Though with the exception of the empty parcel,
C<()>, it is really the comma that creates a Parcel.

    (1 + 2) # not a Parcel
    ()      # empty Parcel
    (1,)    # Parcel with one element
    (1, 2)  # Parcel with two elements

Parcels are immutable, but can contain mutable containers.

    my $x;
    my $p = (0, $x, 2); # can assign to $p[1], but not
                        # to any other element of $p

Word-quoting constructs such as C<< <...> >> also create parcels:

    <a b c> # 3-element Parcel

In flattening list context, parcels are flattened out and disappear:

    my @flat = <a b>, <c, d>;
    say @flat.elems;            # 4

=end pod