Hi all, cleaning up the code in my module, I run into a very interesting problem. I want to pass a user-defined value to Metapost. Until now, the value "scale" had to be numeric, and I used it like this: draw fullcircle scaled \MPvar{scale}mm and Metapost was happy, and I was too. Now Aditya made the reasonable suggestion that users should be allowed to input any dimension and not be restricted to "mm." Easy enough. However, I also want a fallback just in case they input a number, in that case, I want to fall back to mm as a default. So I tried (after Taco's excellent explanation on the wiki): \expandafter\processaction\expandafter[\CVERscale]% [\v!unknown=>{\doifdimensionelse% {\CVERscale}% {\setupMPvariables[CircleSomething][scale=\CVERscale]}% {\doifnumberelse% {\CVERscale}% {\newdimen\CIscale \global\CIscale=\CVERscale mm {\setupMPvariables[CircleSomething][scale=\the\CIscale]}}% {\setupMPvariables[CircleSomething][scale=10mm\relax]% \message{Please give a proper value for the "scale" argument!}}}}, \v!default=>{\setupMPvariables[CircleSomething][scale=10mm \relax]}] So what I want: first check if \CVERscale is a dimension; if it is, assign it to the "scale" parameter. If it isn't, check if it is a number; if this is the case, append the unit "mm" to it and put it into a dimension register. If it isn't a number either, make the value sufficiently absurd for the user to realize and write a message to the terminal. This is all fine and dandy and works, sort of: it works perfectly with dimensions, but when I try with a number as input, it doesn't fall back to the unit "mm," but to "pt," AFAICS. That's nice too, but I have no clue why, and I would prefer mm. If it helps, I can try to give you a minimal test file tomorrow, but maybe somebody spots what I'm doing wrong? (I tried both with mkii and mkiv, and always get the same result.) All best Thomas