Unfortunately, this is very incomplete. It's a bit to basic! You have a nice idea, but maybe instead of "$2 $3 $4", use just $2-4? That's just one suggestion, there could be many for this particular script
I've been working on this nice little calculator here, it's got some functions a simple $calc($2-) doesn't do! I'd like some feedback, how to improve, any features that might be nice, or anything that looks like a bad practice!