Craig is right. Between and Inlist are optimizable without a specific index, except that an index must exist for the fields specified in the between and inlist functions. Empty() doesn't work that way.

For example, between(field1,1,10) will optimize with an index on field1. Inlist(field1,1,2,3,4,5) will optimize with an index on field1. Empty(field1) will not optimize with an index on field1. However, an index on empty(field1) will allow optimization like an index on upper(field) will.