# animation file for a phase-line
permanent
# this code draws a function ff(x)
line [0..99]*.01;ff([j]);[j+1]*.01;ff([j+1]);$RED;2
# a straight line along y=0
line 0;-y0/(y1-y0);1;-y0/(y1-y0);$BLACK
transient
# following the dancing ball
fcircle (x-x0)/(x1-x0);-y0/(y1-y0);.02;$BLUE
end