Confusing Cursor Positions

The wording of some of the cursor positions is a little confusing to 
me. Can someone clarify please? Here is `cursor_after' and 
`cursor_before'

cursor_after (a_cursor: like new_cursor): BOOLEAN is
    -- Is there no valid position to right of `a_cursor'?

This implies to me that if the condition is true, then `a_cursor' 
might be at the last position. If the cursor is at the last position 
then it is at a valid position, but there is no valid position to the 
right.

cursor_before (a_cursor: like new_cursor): BOOLEAN is
    -- Is there no valid position to left of `a_cursor'?

Likewise this implies to me that `a_cursor' might be at the first 
valid position. However `cursor_off' is defined in DS_BILINEAR as

cursor_off (a_cursor: like new_cursor): BOOLEAN is
    -- Is there no item at `a_cursor' position?
  do
    Result := cursor_after (a_cursor) or cursor_before (a_cursor)
  end

which shows that if `a_cursor' is after or before, then it can not be 
at a valid position. This seems confusing to me and it is not clear 
from the contract. In my opinion a better comment for cursor_after 
and cursor_before would be ...

cursor_after (a_cursor: like new_cursor): BOOLEAN is
    -- Is `a_cursor' after all valid items?

cursor_before (a_cursor: like new_cursor): BOOLEAN is
    -- Is `a_cursor' before all valid items?

... or something like that. Do you think my confusion is warranted, 
or is it my lack of understanding?

Thanks,
Brian Heilig






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