Jim,

>> ... !this.value vs this.value=.f.
>
>There is no logical difference between those two expressions. However because ru=shmore requires that an index exist on the expression i the clause, and Rushmore will not use an index with NOT or ! in teh expression, the latter would be otpimizable while the former would not.

Ok, this is an explanation.

>> Another observation, which I found in Help file: EMPTY() is not an optimizable function. So, instead of EMPTY() now I use ''<> myfield.
>
>See my reply to another message. There is nothing about teh empty() function that renders it automatically non optimizable. Its just that the proper indexes must exist for it to be optimized.

Yes, I understand it. Anyway, I believe, I saw list of optimizable functions (and INLIST() was in it) and non-optimizable with EMPTY() as an example. I'll try to find it again and re-read for better understanding. Of course, in most cases it's easy to have index on the field, instead of EMPTY(field), so my first example was at least 85% correct.