-- variant of message at context list:

local template = [[
    path p, q ;
    p := fullcircle scaled 72 ;
    L := length p ;
    N := 20 ;
    q:=subpath (0,%s/N*L) of p ;
    draw q withcolor red ;
    fill fullcircle scaled 3 shifted point length q of q withcolor blue ;
    setbounds currentpicture to unitsquare shifted (-0.5,-0.5) scaled 75 ;
]]

context.starttext()
    for i=0,20 do
        context.startMPpage()
            context(template,i)
        context.stopMPpage()
    end
context.stoptext()
