Do we need $ftypename and $ftypegens?

Right now, [build-deftly] uses this pattern to define a sub-builder field:

  $fname: $< $ftype Builder >,

But I'm fairly sure that won't work if $ftype is generic.

If we had the options, we could say:

  $fname: $< $ftypename Builder> $ftypegens,

But maybe there is a better way to do this.