Scope problem?

Putting force(xl); force(yl) into your interpOne definition (so they get
executed when interpOne is called, not just when the returned function is
called) should work.

*sigh* yes, that looks like it. The help for "force" gives a more
concise example. What I'd posted was actually a cut-down version of my
original problem!

 I've not tested it yet, but other things I'd tried had removed the
bug without me understanding why - such as reassinging the args to
interpOne inside the function.

 Do any other languages work like this?

 Thanks

Barry