From language/setbagmix

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Returns the Baggy addition of its arguments. More information.

From language/operators

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multi sub infix:<(+)>(**@p)

Baggy addition operator.

Returns the Baggy addition of its arguments. This creates a new Bag from each element of the arguments with the weights of the element added together to get the new weight, if none of the arguments are a Mix or MixHash.

say <a a b c a d> (+) <a a b c c>; # OUTPUT: «Bag(a(5) b(2) c(3) d)␤» 

If any of the arguments is a Mixy, the result is a new Mix.

say <a b c> (+) (a => 2.5, b => 3.14).Mix; # OUTPUT: «Mix(a(3.5) b(4.14) c)␤»