Identity Preservation in Hakha-Lai Tone Sandhi

This paper examines the tone sandhi phenomenon in the Tibeto-Berman language of Hakha-Lai, which is special for the complex interactions among its elementary rules. The rule interactions in Hakha-Lai lead to both left-to-right and right-to-left rule directionalities in sequences of three or more tones. The rule application directionalities, however, appear to be ungoverned, as none of the principles proposed to date that may contribute to determining directionalities can account for them. In this paper, I argue that the tone sandhi operation directionalities in Hakha-Lai are by no means ungoverned. Normally tone sandhi operates from right to left for identity reasons. This is forced by the IDENT-BOT constraint. The right-to-left direction is sacrificed only when such direction would result in output forms that contain marked sequences or tonal changes at the prominent position, which are forbidden by U[AGREE-t and IDENT-IO-T-L respectively. Thus, the directionalities of tone sandhi operation in Hakha-Lai are naturally predicted by the interaction of the IDENT-BOT constraint, the U[AGREE-t constraint, and the IDENT-IO-T-L constraint, where IDENT-BOT must be dominated by the latter two constraints.