refer @varsha394 comment for knowing about exportation law. Now let's see its application in programming.
//(if X then Y) unless Z
//(if X then Y) if not Z
//(if X then Y) if ~Z
//if ~Z then(if X then Y)
if(Z is not true)
{
if(X is true)
{
print("Y is true") //Y is true
}
}
//if X then (Y unless Z)
//if X then (Y if not Z)
//if X then (Y if ~Z)
//if X then (if ~Z then Y)
if(X is true)
{
if(Z is not true)
{
print("Y is true") //Y is true
}
}
//By Exportation law,we can write
//if X^~Z then Y
// X^~Z -> Y
if(X is true and Z is not true)
{
print("Y is true")//Y is true
}