我有一个带有属性的类。我想检查是否定义了一些但不是全部。所以：
class A {
has $.a is rw;
has $.b is rw;
has $.c is rw;
has $.d is rw;
method delete { ... }
}
my A $x .= new(:a<hi>, :d<good>);
## later
$x.b = 'there';
## code in which $x.c may or may not be defined.
## now I want to check if the attributes a, b, and c are defined, without
## needing to know about d
my Bool $taint = False;
for <a b c> { …