(t x) is not a name is_cast knows - t is not a machine type - so it is
its own arm, admitted by numeric? because a cast produces a number.
vec-new, pool-new and map-new all reach the one list of what names a
type, so the spike's line for vec-new had already covered the other two;
zeroed takes its type from the position it is written in. All four are
pinned in programs/generics.flan.