this is allowed because 'nat' now appears only at the end of 'npair' and there it
  (a) does not need to consume leading whitespace and
  (b) no longer needs to test for a trailing delimiter

this is allowed because 'pnat' now only remains at the beginning of 'npair'
and there
  (a) does not need to consume leading whitespace and
  (b) is explicitly delimited by 'wel'

the code would still try to print a last element.

also rearrange the loop to avoid code duplication and use delta-1
spaces after the comma.

/ht andrea