Forums Scripts & Popups Compact If Statement

I try to do the following:

if $len(%pos = $pos(%msg, mother)) != 0 {
/echo someone says mother at %pos
} else if $len(%pos = $pos(%msg, father)) != 0 {
/echo someone says father at %pos
}

But the script does not seem to work. I think because I am assigning a value to %pos and at the same time sending the value over to the function $len(). How do I rectify this?