It is often useful to add $(warning /$0/ /$1/ /$2/ /$3/) at the very
start of each interesting macro.

Recal that foreach does nothing when the list only contains spaces,
and adds spaces between the results even if some results are empty.

If debugging the reader:
# export READER_DEBUG=1

In order to get the equivalent of DEBUG_EVAL in step2:
# export EVAL_DEBUG=1