when a "SET" statement is accidentally put into a SEQUENCE, it is emitted to the acf file without compilation or warning

a SET statement is legal only in WAVEFORM blocks (and not in SEQUENCE blocks). However, if you make a sequence block with a set statement, e.g. 

SEQUENCE BrokenSequence {
   SET [12:1 , 12:2] to 1;
}

there is no warning issued, wdl happily emits these lines exactly into the resulting acf file (even though this [ ] notation is not in fact legal in an acf file).

