title: Perl 6: how to check `new` for invalid arguments?
body: <p>What is the simplest way to check if invalid arguments are passed to the constructor method <code>new</code>?</p>
  
  <pre><code>use v6;
  unit class Abc;
  
  has Int $.a;
  
  my $new = Abc.new( :b(4) );
  </code></pre>