Basic messages are unhelpful. Example::

if Inc<Lvls(i)then

	--> error msg:
Inadmissible value or data type.
Index out of defined range
	...vague!
What variable is at fault? Inc, i, or Lvls?
What is inadmissible: a value, or a data type?
Index... I guess it's 'i'; what was its value when the error occurred?
defined range... what is its value? --eg, 1...9?
	Of course, a brief msg is enough for the basic expert, who quickly sees what
the problem is. But the unexperienced user learns from his/her errors...
provided the error messages are clear and specific.