hi,
you can use two different ways:
1) with 'mul2 x,y' you can stretch the things behind the command. you can use line2, circle2 or also the fragment2 command. of course the circle will get an ellips, if x and y are not identical. if typical values for a is 2.0 and for b 1.2 then you can script: mul2 a/2.0,b/1.2 ...
2) you can define the elements in dependency to a and b:
line2 0,0,a,0
line2 0,0,0,b
This way is more difficult but sometimes you need more control over the elements.
best regards. gerd