start and end expressions determine animation positions for each particle, on a 0..1 scale.

r() yields a number from the random source.

Example expressions 
      r()
      Math.sqrt(r())
      1- r()
      1- r()*r()
      1- r()*r()*r()
      1- r()*r()*r()*r()
      Math.sqrt(1- r()*r()*r()*r())
      1 - Math.sqrt(1- r())
      1 - Math.sqrt(1- r() * r())
ixfx