Two positions are in one chain when they hold one term.
Used by LinearizedSyntacticObject.chainRel_iff, LinearizedSyntacticObject.instDecidableRelPositionsVertexTokenValRoseTreeIsSyntacticObjectMkR, LinearizedSyntacticObject.card_chainRel, LinearizedSyntacticObject.exists_chainRel_of_self_mem_copyRel, LinearizedSyntacticObject.self_mem_copyRel_iff