Substr with a negative length

 What should the substr ops produce if given a negative length argument?
 At the moment, the four-arg. form hands back an empty string, while
 the five-arg. form hands back a copy of the original string, ie:

  set S0, "abcdefg"
  substr S1, S0, 0, -1
  print S1
  print "\n"

 and

  set S2, ""
  substr S3, S0, 0, -1, S2
  print S3
  print "\n"

 produce "" and "abcdefg" respectively. The former seems more correct,
 but I thought I'd throw this out for comment before hacking string.c

 Simon

• Substr with a negative length by Simon Glover