AppleScript tip: if/then syntax

I showed some basic AppleScript boolean-oriented syntax like this in another post:

set a to true
if a then
  display dialog a
end if

That code doesn't do too much, especially because you know the variable a is set to true, but it becomes a little more helpful when you don't know if a is true or false:

if a then
  -- do something really important here
end if

You can perform other if/then checks based on numerical tests, like this:

set balance to 0
if balance <= 0 then
  display dialog "No money left in the bank account"
end if

Finally, you can combine boolean tests using syntax like this:

set checkingBalance to 0
set savingsBalance to 0
if checkingBalance <= 0 and savingsBalance <= 0 then
  display dialog "Yikes!"
end if