The verb sa is used to mean 'have the habit of', and thus functions as a habitual marker. It presumably evolved as a special use of the verb sa 'to know'.
Examples 97, 16, 4 and 96 provided under Value 9 illustrate the varying interpretations of an utterance absent a habitual or imperfective marker. In such cases, it is context which determines whether the utterance is interpreted as habitual, current state, progressive, (immediate) future, a.o.
Source: Kouwenberg 2007: 329
Source: Kouwenberg nd
Source: nd: 7 July 2009, p.5
Source: Kouwenberg and Muysken 1995: 210
Source: Maurer 1988: 123
Source: Kouwenberg nd