A rule that disallows the use of an if or unless with a negated predicate. If you do this, swap the if for an unless, or vice versa.

These are considered valid:

if happy? do
  party()
end
unless sad? do
  jump_up()
end

These are considered invalid:

if !happy? do
  stay_in_bed()
end
unless not sad? do
  mope_about()
end