Joins tokens to elements on equality of :join_bind. The common case: no guard reaches across conditions, so the join is a hash lookup.
:join_bind
@type t() :: %Rete.Network.Node.HashJoin{ alpha_code: atom(), fact_binding: atom() | nil, id: non_neg_integer() | nil, join_bind: [atom()], new_bind: [atom()], type: atom() }