[Therion] scaling a line thickness in Metapost

I want to be able to select a different thickness of line for different "scale" values that can be selected in the .th2 file. Lines can have a scale like this, which seems perfect for what I need: -scale xs * Is there a way to access this value from within a custom line definition like this? def l_u_foo (expr P)= if ATTR_scale ... * Assuming it can be seen from some ATTR value, internal variabe or function parameter, what values will it see for the various -scale options (xs, s, m, l, xl) - if it is a number, what are the numeric values for the various scales, or if it is a string, does it just see the literal values "xs", "s" etc.? Cheers! Tarquin