vardef mycircle =
  save c ; picture c ; c := nullpicture ;
  addto c contour fullcircle scaled 2mm ;
  c
enddef ;

def a expr p =
  path _p_ ; _p_ := p ;
  a_impl
enddef ;

def a_impl text t =
  begingroup
    save c ; picture c ; c := mycircle ;
    clip c to bbox c scaled 0.5 ;
    addto currentpicture also c t ;
    addto currentpicture doublepath _p_ t ;
  endgroup
enddef ;

def b expr p =
  path _p_ ; _p_ := p ;
  b_impl
enddef ;

def b_impl text t =
  begingroup
    save c; picture c ; c := mycircle ;
    addto currentpicture also c t ;
    addto currentpicture doublepath _p_ t ;
  endgroup
enddef ;
beginfig(1)

  a (0,0) -- (1cm,0) dashed evenly ;
  currentpicture := currentpicture shifted (2cm,0) ;
  a (0,0) -- (1cm,0) ;

  currentpicture := currentpicture shifted (-2cm, 0.5cm );

  b (0,0) -- (1cm,0) dashed evenly ;
  currentpicture := currentpicture shifted (2cm,0) ;
  b (0,0) -- (1cm,0) ;

endfig ;
end