drawDoubleLine in feynman module draws invisible arrows

[a-maier]

import feynman;

drawDoubleLine(40E--40W, MidArrow);

is supposed to draw a double line with an arrow. However, the arrow is invisible. The culprit is the last line of drawDoubleLine:

  draw(pic, p, invisible, arrow);

The intent is apparently to draw an invisible line with a visible arrow. In reality both are invisible.