From type/Metamodel/MultipleInheritance

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method parents($obj, :$all, :$tree)

Returns the list of parent classes. By default it stops at Cool, Any or Mu, which you can suppress by supplying the :all adverb. With :tree, a nested list is returned.

class D { };
class C1 is D { };
class C2 is D { };
class B is C1 is C2 { };
class A is B { };
 
say A.^parents(:all).raku;
# OUTPUT: «(B, C1, C2, D, Any, Mu)␤» 
say A.^parents(:all, :tree).raku;
# OUTPUT: «[B, ([C1, [D, [Any, [Mu]]]], [C2, [D, [Any, [Mu]]]])]␤»