labeling points in metafont's proofs?

Question

I am following the http://metafont.tutorial.free.fr/ tutorial, where labels is used to show given points in the proofs. I am not able to get them in my case. Can someone help?

mg:=1.5;
mode_setup;
tracingtitles:=1;
height#:=mg*9.6pt#;
midheight#:=mg*4pt#;
depth#:=mg*5pt#;
fullwidth#:=mg*3pt#;
pensize#:=mg*.3pt;
define_pixels(height,midheight,depth,fullwidth)

% points used for curves, and I also will have them labeled 
pair pAA,pBB,pCC;
pAA:=(0,midheight);
pBB:=(0,0);
pCC:=(fullwidth,0);

beginchar("a",fullwidth#,midheight#,0); "a";
    pickup pencircle scaled pensize#;
    draw pAA .. pBB .. pCC;
    draw pCC -- pAA;
    %% label points here -- does not work :-(
    labels(pAA,pBB,pCC);
endchar

end;

after running mf foo.mf, gftodvi foo.2602gf, the foo.dvi looks like this (no points shown):

EDIT: Besides the answers by egreg below, labeled points can be shown with makelabel("label",point):

    makelabel("AA",pAA);
    makelabel("BB",pBB);
    makelabel("CC",pCC);

produces

Answer 1

The instruction label takes as argument a list of suffixes, not of pairs; for example, labels(1) will show a dot and a 1 corresponding to point z1.

mg:=1.5;
mode_setup;
tracingtitles:=1;
height#:=mg*9.6pt#;
midheight#:=mg*4pt#;
depth#:=mg*5pt#;
fullwidth#:=mg*3pt#;
pensize#:=mg*.3pt;
define_pixels(height,midheight,depth,fullwidth)

% points used for curves, and I also will have them labeled

beginchar("a",fullwidth#,midheight#,0); "a";
z1=(0,midheight);
z2=(0,0);
z3=(fullwidth,0);
pickup pencircle scaled pensize#;
draw z1 .. z2 .. z3;
draw z3 -- z1;
%% label points here
labels(1,2,3);
endchar;

end;