Our existing function nullifeq can be used to exclude single values from the base, For example nullifeq(Q1,99) excludes 99 from the base. But we need functions to exclude ranges from the base, e.g.:

nullifgt(nbchildren, 10) -- exclude records where nbchildren > 10
nullifgte(nbchildren, 10) -- exclude records where nbchildren >= 10
nulliflt(income, 3) -- exclude records where income < 3
nulliflt(income, 3) -- exclude records where income <= 3
nullifwithin(fred, 10, 20) -- exclude records where 10 <= fred <= 20