gistfile1.txt

module Foo { 
  my class SingleInheritanceClassHOW
  is Metamodel::ClassHOW
  {
    method add_parent(Mu $obj, Mu $parent) {
      if +self.parents($obj, :local) > 0 {
        die "Multiple inheritance is forbidden!";
      }
      callsame;
    }
  } 
  my module EXPORTHOW { }
  EXPORTHOW.WHO.<class> = SingleInheritanceClassHOW;
}

import Foo;

class A is Int is Str { };