Fractional beams (beamlets) point toward the note(s) they group with

A partial beam on an isolated shorter note within a group points toward whichever neighboring note it forms a sub-group with (e.g., in 6/8's eighth-sixteenth-eighth-sixteenth, the beamlet direction implies whether the rhythm reads as three eighth-groups or two three-sixteenth-groups).

A beamlet points away from the note it rhythmically groups with, implying the wrong subdivision reading.

VexFlow's own doc comment on `Beam.setPartialBeamSideAt` cites this exact convention with an ASCII diagram; Gould, Behind Bars, ch. Beaming discusses beamlet direction as meaningful notation, not decoration.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.