I am very reserved about the classification of ba as a verb. To me, it is rather a preposition since commutation with prepositions is possible.
Source: Fattier 1998: vol. 3, p. 455
Source: Valdman et al. 1996: 152
Source: Valdman et al. (eds.) 2007: 72
Source: Desmarattes 1983: 51