How do I distinguish between true, false, and null?

Add this dictionary to your group file:

isFalse ::= ["false":"true"]

It returns true if the value is false, and null otherwise. Then you can use it in 'if statements'.

main(nullValue, falseValue) ::= <<
$if(isFalse.(falseValue))$
    falseValue value is false;
$endif$

$if(!isFalse.(nullValue))$
    nullValue is NOT false;
$endif$
>>