User-defined Functions

I've created a simple function.
function PCT200 ()
{
result = ( Close / MA( Close, 200 ) ) - 1;
return result;
}
How do I use this function? Everytime I try to use it, it tells me the function is not defined. What am I doing wrong?

function PCT200 ()
{
result = Ref( Close / MA( Close, 200 ) ) - 1;
return result;
}