Some formats (notably dvi) have records containing an integral field, where the width of that field is determined by the record identifier.
It would be most convenient to be able represent this for example as:
deftype seti = struct
int<8> opcode: opcode >= 128 && opcode < 131;
int<(opcode - 128) * 8> character;
Other ways to represent this are possible, but they all require either a lot
more struct definitions or a lot of unions and conditionals.
I'm closing this enhancement request, as we are maintaining them in the POKE recfile that you can find in the source distribution, not here.
But don't worry, this issue is listed there :)