It's unclear whether ProtoSMILES allows a virtual hydrogen count on hydrogen itself. Weininger notes that "explicit hydrogen specification is required" in the case of "hydrogens connected to other hydrogens, e.g. [H][H], molecular hydrogen" . This may reflect an erroneous belief that parsers would be incapable of processing the implied strings (e.g., "[HH]"). To be clear, Balsa supports virtual hydrogen counts on all atoms.