sub f(
            $self isa Foo::Bar,         # croak unless $self->isa('Foo::Bar');
            $foo  isa Foo::Bar?,        # croak unless undef or of that class
            $a!,                        # croak unless $a is defined
            $b    is  Int,              # croak if $b not int-like
            $c    is  Int?,             # croak unless undefined or int-like
            $d    is PositiveInt,       # user-defined type
            $e    is Int where $_ >= 1, # multiple constraints
            $f    is \@,                # croak unless  array ref
            $aref as ref ? $_ : [ $_ ]  # coercions: maybe modify the param
    ) { ...};