insn semantics required?

[Nathan Froyd]

I've been looking through the documentation and couldn't determine
conclusively whether defining the semantics of an insn is required.  Is
it?  Auto-magically defining a simulator seems less useful in this day
and age, but an auto-generated assembler and disassembler seems quite
useful.

I suppose as a stopgap one could simply provide an uninteresting
semantics for each insn.

-Nathan