continuation
A veto asked at every step boundary: may the vehicle go on past this one?
What a guide path's movement gate is, on a substrate whose decision points are interpolation steps rather than zone boundaries. Answering false stops the vehicle where it stands, exactly as halt does -- so a flat battery or a breakdown is discovered part way through a journey rather than at the end of it, which is the whole reason a substrate has decision points at all. Null asks nothing, which is the default and costs nothing.