Actions.pm

role FoldArithmetic::Actions does PAST::Optimization::Actions {
    method foldAdd($/) {
	my ($left, $right) := $<PAST::Val>;
	my $result := PAST::Val.new(:value(pir::add($left.value(), $right.value())));
	make $result;
    }
}

Optimization.pm

grammar FoldArithmetic::Optimization does PAST::Optimization {
    rule foldAdd {
	<PAST::Op(<PAST::Val>, <PAST::Val>)>
    }
}