Simple constraint

If $x_{1}$ follows $x_{0}$, then $x_{1}$ gets its coordinates from the following set of equations:

Calculate the lenth difference

$l={\sqrt {(px_{1}-px_{0})^{2}+(py_{1}-py_{0})^{2}}}$

, and how much to add with given radius, $r$

$dx={\frac {px_{1}-px_{0}}{l}}*r-px_{1}+px_{0}$

, where

$p_{1}=p_{1}+d$

is the new point.