[metapost] Extending a path

[Yasir Malik]

> Or if you want to extend pp with a straight line for
> a distance d:
>  path qq;
>  n := length pp;
>  qq := (0,0)--(d*unitvector(direction n of pp));
>  pp & (qq shifted point n of pp)
>
Why is it in the following program, the commented out code gives the error 
"! Paths don't touch; `&' will be changed to `..'." but the code above it 
works fine?
beginfig(1);

u = 1cm;

path qq, pp;
numeric n, d;

pp := (3u, 4u)--(8u, 10u);

d := 1u;
n := length pp;

pickup pencircle scaled 4pt;
drawdot(point 0 of pp);
drawdot(point n of pp);

pickup pencircle scaled 1pt;

qq := (0,0)--(d*unitvector(direction n of pp));
draw pp & (qq shifted point n of pp);

%qq := (0, 0)--(-d*unitvector(direction n of pp));
%draw pp & (qq shifted point 0 of pp);

endfig;
end


If I just draw the following:
qq := (0, 0)--(-d*unitvector(direction n of pp));
draw (qq shifted point 0 of pp);

the path correctly appears at the point 0=(3, 4) of pp.  If I change the & 
to --, I notice the line produced is slightly more jagged than path 
produced by draw pp & (qq shifted point n of pp);

Thanks,
Yasir