[plt-dev] slightly misleading error

> How about:
> 
> "the dynamic context of this expression expected 1 value, but this
> expression produced 2 values instead: a b"

Fine with me, but one question: are there situations where this same 
message will be produced but the producer expression *won't* be the one 
highlighted? For example, if the producer expression is bytecode-compiled?

If so, there should either be two different error messages for the 
different situations, or the English has to be very carefully chosen to 
make sense in both cases.

Dave