% Copyright (c) 2026 Simon Rubinstein-Salzedo.
% Distributed under the LaTeX Project Public License, version 1.3c or later.
% Maintenance status: maintained. Current Maintainer: Simon Rubinstein-Salzedo.
% License: https://www.latex-project.org/lppl.txt
% See README.md for the list of files and attribution.
% Adapted from the combgames symbol designs by Raymond Chen,
% David Wolfe and Aaron N. Siegel; this is a renamed, modified source.
prologues := 3;
% outputtemplate := "%j-%c.mps";
outputformat := "svg";        % MetaPost 2.x
outputtemplate := "%j-%c.svg";  % e.g., mysyms-15.svg, -16.svg, ...


% Keep the nominal layout box, but never exclude ink (including pen width).
% This also handles reflected pictures and strokes below the baseline.
def fitbounds(expr nominal) =
  begingroup
    save bboxmargin, ink, lo, hi;
    bboxmargin := 0;
    path ink; ink := bbox currentpicture;
    pair lo, hi;
    lo := (min(xpart llcorner ink, xpart llcorner nominal),
           min(ypart llcorner ink, ypart llcorner nominal));
    hi := (max(xpart urcorner ink, xpart urcorner nominal),
           max(ypart urcorner ink, ypart urcorner nominal));
    setbounds currentpicture to
      (lo -- (xpart hi, ypart lo) -- hi -- (xpart lo, ypart hi) -- cycle);
  endgroup
enddef;

beginfig(0);

  % --- CM10-ish parameters (10pt) ---
  u          := 10/18pt;            % ≈0.55556 pt
  w          := 12u;                % glyph width (like MF)
  x_height   := 4.30556pt;          % cmr10 ex
  asc_height := 6.94444pt;          % cap/asc height
  asc_depth  := 2.0pt;

  rule_thickness := 0.40pt;         % bar thickness
  spread := 0.45*x_height;          % half spacing center↔outer bar

  % Arrowhead geometry
  bar_frac := .4;
  head_frac := 0.48;                % head height as fraction of asc_height
  overhang  := 2u;                  % how far the head base extends beyond each outer bar

  % --- derived geometry ---
  numeric h, d, xmid, ybase;
  h := asc_height;
  d := asc_depth;
  xmid := .5w;
  ybase := h - head_frac*asc_height;
  barbase := h - bar_frac*asc_height;

  % bar x-positions
  numeric xl, xc, xr;
  xl := xmid - spread;  xc := xmid;  xr := xmid + spread;

  % Draw the three stems with round ends; the middle stem reaches the apex.
  pickup pencircle scaled rule_thickness;
  draw (xl,-d)--(xl,barbase);
  draw (xc,-d)--(xc,h);
  draw (xr,-d)--(xr,barbase);

  % Stroked, curved arrowhead using MF's golden-ratio control anchor
  pair A, BL, BR, M, Znine;
  A  := (xmid, h);                       % apex
  BL := (xl - overhang, ybase);          % base corners OUTSIDE the outer bars
  BR := (xr + overhang, ybase);
  M  := .5[BL, BR];
  Znine := 0.381966[M, A];               % MF used 0.381966 (=1/phi^2)

  path leftedge, rightedge;
  leftedge  := A .. {BL - Znine} BL;
  rightedge := A .. {BR - Znine} BR;

  draw leftedge;
  draw rightedge;

  fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);

  picture TRIPLEUP; TRIPLEUP := currentpicture;

endfig;

beginfig(1);

  draw TRIPLEUP yscaled -1;
fitbounds((0,-h)--(w,-h)--(w,d)--(0,d)--cycle);

endfig;

beginfig(2);

  % --- CM10-ish parameters (10pt) ---
  u          := 10/18pt;            % ≈0.55556 pt
  w          := 12u;                % glyph width (like MF)
  x_height   := 4.30556pt;          % cmr10 ex
  asc_height := 6.94444pt;          % cap/asc height
  asc_depth  := 2.0pt;

  rule_thickness := 0.40pt;         % bar thickness
  spread := 0.3*x_height;          % half spacing center↔outer bar

  % Arrowhead geometry
  bar_frac := .4;
  head_frac := 0.47;                % head height as fraction of asc_height
  overhang  := 2u;                  % how far the head base extends beyond each outer bar

  % --- derived geometry ---
  numeric h, d, xmid, ybase;
  h := asc_height;
  d := asc_depth;
  xmid := .5w;
  ybase := h - head_frac*asc_height;
  barbase := h - bar_frac*asc_height;
  h1 := .45*ybase + .55*h;	

  % bar x-positions
  numeric xl, xc, xr;
  xl := xmid - 1.5spread;  xl1 := xmid - .5spread;  xr1 := xmid + .5spread; xr := xmid + 1.5spread;

  % Draw four stems with round ends; the inner pair reaches higher.
  pickup pencircle scaled rule_thickness;
  draw (xl,-d)--(xl,barbase);
  draw (xl1,-d)--(xl1,h1);
  draw (xr1,-d)--(xr1,h1);
  draw (xr,-d)--(xr,barbase);

  % Stroked, curved arrowhead using MF's golden-ratio control anchor
  pair A, BL, BR, M, Znine;
  A  := (xmid, h);                       % apex
  BL := (xl - overhang, ybase);          % base corners OUTSIDE the outer bars
  BR := (xr + overhang, ybase);
  M  := .5[BL, BR];
  Znine := 0.381966[M, A];               % MF used 0.381966 (=1/phi^2)

  path leftedge, rightedge;
  leftedge  := A .. {BL - Znine} BL;
  rightedge := A .. {BR - Znine} BR;

  draw leftedge;
  draw rightedge;

fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);

  picture QUADUP; QUADUP := currentpicture;

endfig;

beginfig(3);
  draw QUADUP yscaled -1;
  fitbounds((0,-h)--(w,-h)--(w,d)--(0,d)--cycle);
endfig;


beginfig(15);

  % CM10-like parameters at 10pt
  u := 10/18pt;                 % ≈ 0.55556 pt
  w := 17u;                     % ≈ 9.444 pt
  x_height := 4.30556pt;        % cmr10 x-height
  spread := 1.25*x_height;      % ≈ 5.382 pt
  rule_thickness := 0.40pt;     % TeX default rule thickness

  pickup pencircle scaled rule_thickness;

  % --- exact MF relations ---
  x2 := w - 1.5u;   y2 := 0;                % tip
  x1 := w - x2 + 3u;  x3 := x1;             % base line
  y1 :=  .5*spread;    y3 := - .5*spread;

  x4 := x1 - 3u;  x5 := x4;                 % vertical bar
  y4 := y1;       y5 := y3;

  draw z1--z2--z3--cycle;                   % triangle outline
  draw z4--z5;                              % vertical bar

  picture GFUZ; GFUZ = currentpicture;

  fitbounds((0,-.5spread)--(w,-.5spread)--(w,.5spread)--(0,.5spread)--cycle);
endfig;

beginfig(16);
  draw GFUZ xscaled -1;
  fitbounds((-w,-.5spread)--(0,-.5spread)--(0,.5spread)--(-w,.5spread)--cycle);
endfig;

beginfig(17);

  % CM10-like parameters at 10pt
  u := 10/18pt;                 % ≈ 0.55556 pt
  w := 17u;                     % ≈ 9.444 pt
  x_height := 4.30556pt;        % cmr10 x-height
  spread := 1.25*x_height;      % ≈ 5.382 pt
  rule_thickness := 0.40pt;     % TeX default rule thickness

  pickup pencircle scaled rule_thickness;

  % --- exact MF relations ---
  x2 := w - 1.5u;   y2 := 0;                % tip
  x1 := w - x2 + 3u;  x3 := x1;             % base line
  y1 :=  .5*spread;    y3 := - .5*spread;

  x4 := x1 - 3u;  x5 := x4;                 % vertical bar
  y4 := y1;       y5 := y3;

  draw z1--z3;                   	    % vertical bar
  draw z4--z5;                              % vertical bar
  fitbounds((0,-.5spread)--(w,-.5spread)--(w,.5spread)--(0,.5spread)--cycle);
endfig;

beginfig(30);

  u          := 10/18pt;            % ≈0.5556 pt
  x_height   := 4.30556pt;
  h          := 0.90*x_height;      % height above axis
w := 2*h + 2*u;   % makes horizontal length = vertical length = 2h
  d          := h;                  % symmetric depth
  math_axis  := 0;

  % --- key knobs (make ends BIG, center thin) ---
  flare := 0.15*w;                  % end half-thickness driver (≈ 2 pt if w≈5.6 pt)
  stem  := 0.50pt;                  % center stroke thickness before the 1.1 factor

  % nodes per MF relations
  z1 = (0.5w,  h);     z2 = (0.5w, -h);     % vertical ends
  z5 = (0.5w,  math_axis);                  % vertical mid
  z6 = (0.5w,  math_axis);                  % horizontal mid
  z3 = (u,     math_axis);                  % left end
  z4 = (w-u,   math_axis);                  % right end

  % variable-width outlines (MF: pos1(2flare,0), pos5(1.1stem,0), etc.)
  numeric fl, st; fl := 2*flare; st := 1.1*stem;

  % vertical stroke offsets (±x)
  z1l = (x1 - .5*fl, y1);  z1r = (x1 + .5*fl, y1);
  z5l = (x5 - .5*st, y5);  z5r = (x5 + .5*st, y5);
  z2l = (x2 - .5*fl, y2);  z2r = (x2 + .5*fl, y2);

  % horizontal stroke offsets (±y)
  z3l = (x3, y3 - .5*fl);  z3r = (x3, y3 + .5*fl);
  z6l = (x6, y6 - .5*st);  z6r = (x6, y6 + .5*st);
  z4l = (x4, y4 - .5*fl);  z4r = (x4, y4 + .5*fl);

  % build the two stroked shapes and fill them (like MF's filldraw stroke ...)
  path Vleft, Vright, Hleft, Hright;
  Vleft  := z1l .. z5l .. z2l;
  Vright := z1r .. z5r .. z2r;
  Hleft  := z3l .. z6l .. z4l;
  Hright := z3r .. z6r .. z4r;

  fill Vleft -- reverse Vright -- cycle;
  fill Hleft -- reverse Hright -- cycle;

  % bounding box like beginarithchar (so scaling behaves)
  fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);

endfig;

beginfig(31);

  u          := 10/18pt;            % ≈0.5556 pt
  x_height   := 4.30556pt;
  h          := 0.90*x_height;      % height above axis
w := 2*h + 2*u;   % makes horizontal length = vertical length = 2h
  d          := h;                  % symmetric depth
  math_axis  := 0;

  % --- key knobs (make ends BIG, center thin) ---
  flare := 0.15*w;                  % end half-thickness driver (≈ 2 pt if w≈5.6 pt)
  stem  := 0.50pt;                  % center stroke thickness before the 1.1 factor

  % nodes per MF relations
  z1 = (0.5w,  h);     z2 = (0.5w, -h);     % vertical ends
  z5 = (0.5w,  math_axis);                  % vertical mid
  z6 = (0.5w,  math_axis);                  % horizontal mid
  z3 = (u,     math_axis);                  % left end
  z4 = (w-u,   math_axis);                  % right end

  % variable-width outlines (MF: pos1(2flare,0), pos5(1.1stem,0), etc.)
  numeric fl, st; fl := 2*flare; st := 1.1*stem;

  % vertical stroke offsets (±x)
  z1l = (x1 - .5*fl, y1);  z1r = (x1 + .5*fl, y1);
  z5l = (x5 - .5*st, y5);  z5r = (x5 + .5*st, y5);
  z2l = (x2 - .5*fl, y2);  z2r = (x2 + .5*fl, y2);

  % horizontal stroke offsets (±y)
  z3l = (x3, y3 - .5*fl);  z3r = (x3, y3 + .5*fl);
  z6l = (x6, y6 - .5*st);  z6r = (x6, y6 + .5*st);
  z4l = (x4, y4 - .5*fl);  z4r = (x4, y4 + .5*fl);

  % build the two stroked shapes and fill them (like MF's filldraw stroke ...)
  path Vleft, Vright, Hleft, Hright;
  Vleft  := z1l .. z5l .. z2l;
  Vright := z1r .. z5r .. z2r;
  Hleft  := z3l .. z6l .. z4l;
  Hright := z3r .. z6r .. z4r;

  fill Hleft -- reverse Hright -- cycle;

  % bounding box like beginarithchar (so scaling behaves)
  fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);

endfig;

beginfig(40);

  u := 10/18pt;            % ≈0.5556 pt
  w := 12u;                % glyph width
  x_height := 4.30556pt;
  h := 0.90*x_height;      % height above axis
  d := h;                  % symmetric depth (arith char)
  rule_thickness := 0.40pt;

  pickup pencircle scaled rule_thickness;

  % points (MF relations)
  z0 = (.5w, 0);           % math axis
  z1 = (.5w,  h);          % top of stem
  z2 = (.5w, -h);          % bottom of stem

  x3 := u;       y3 := -0.5h;   % y3 = 0.5[y0,y2] with y0=0, y2=-h
  x4 := w - x3; y4 := y3;
  z3 = (x3, y3);
  z4 = (x4, y4);

  draw z1--z2;             % stem
  draw z3--z0--z4;         % V-shaped crossbar

  fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);

  picture UPSUM; UPSUM = currentpicture;

endfig;

beginfig(41);
  draw UPSUM yscaled -1;
  fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);
endfig;

beginfig(50);

  % --- metrics (CM10-ish defaults; tweak as you like) ---
  u           := 10/18pt;            % ~0.5556 pt
  w           := 18u;                % glyph width (from beginchar(...,18u,...))
  asc_height  := 6.94444pt;          % cap height
  desc_depth  := 2.0pt;              % descender depth
  x_height    := 4.30556pt;          % ex
  bar_height  := 0.50*x_height;      % MF used 'bar_height' (tweak if needed)
  rule_thickness := 0.40pt;          % stroke for inner lines
  circle_factor := 1.5;              % MF drew circle with 1.5*rule_thickness

  % --- circle geometry (make it a true circle touching the 4 extremes) ---
  % MF set: left x6 = u, right x2 = w-u; top y8 = h+o; bottom y4 = -d-o
  % Choose 'o' so horizontal and vertical diameters match (a true circle):
  h := asc_height;  d := desc_depth;
  o := 0.5*((w - 2u) - (h + d));     % may be slightly negative; that’s fine

  pair C; numeric R;
  C := (.5w, .5*(h - d));            % circle center
  R := .5*(w - 2u);                  % radius ( = .5*(h+d+2o) by construction)

  % outer circle (thicker pen, like 'pencircle scaled 1.5rule_thickness')
  pickup pencircle scaled (circle_factor*rule_thickness);
  draw fullcircle scaled (2R) shifted C;

  % --- inner strokes (round pen, matching the other line symbols) ---
  pickup pencircle scaled rule_thickness;

  % MF: lft x9 = good.x (.5w - 2u); x9 = x10 = x15;  x11 = x12 = w - x9
  x9  := .5w - 2u;   x10 := x9;  x15 := x9;
  x11 := w - x9;     x12 := x11;

  % MF: y10=y12=.5h-6u; y9=.5h+2u;
  y10 := .5h - 6u;   y12 := y10;
  y9  := .5h + 2u;

  % MF: y15=y11=(x_height/asc_height)[y10,y9];
  y11 := (x_height/asc_height)[y10,y9];
  y15 := y11;

  % points
  z9  = (x9,  y9);
  z10 = (x10,y10);
  z11 = (x11,y11);
  z12 = (x12,y12);
  z15 = (x15,y15);

  % MF: z13 = (.7*bar_height/asc_height)[z10,z9];
  z13 = (.7*bar_height/asc_height)[z10,z9];

  % MF: z14 = whatever[z11,z13] = whatever[z12,z15];
  % (intersection of the two infinite lines through those pairs)
z14 = whatever[z11,z13] = whatever[z12,z15];

  % draw the three segments
  draw z9--z10;      % short vertical on the left
  draw z13--z11;     % upper diagonal to the right
  draw z14--z12;     % lower diagonal to the right

  % explicit bounding box (so scaling/clipping behaves like beginchar)
  fitbounds((0,-d - o)--(w,-d - o)--(w,h + o)--(0,h + o)--cycle);

  picture KO; KO = currentpicture;

endfig;

beginfig(51);

  % --- metrics (CM10-ish defaults; tweak as you like) ---
  u           := 10/18pt;            % ~0.5556 pt
  w           := 18u;                % glyph width (from beginchar(...,18u,...))
  asc_height  := 6.94444pt;          % cap height
  desc_depth  := 2.0pt;              % descender depth
  x_height    := 4.30556pt;          % ex
  bar_height  := 0.50*x_height;      % MF used 'bar_height' (tweak if needed)
  rule_thickness := 0.40pt;          % stroke for inner lines
  circle_factor := 1.5;              % MF drew circle with 1.5*rule_thickness

  % --- circle geometry (make it a true circle touching the 4 extremes) ---
  % MF set: left x6 = u, right x2 = w-u; top y8 = h+o; bottom y4 = -d-o
  % Choose 'o' so horizontal and vertical diameters match (a true circle):
  h := asc_height;  d := desc_depth;
  o := 0.5*((w - 2u) - (h + d));     % may be slightly negative; that’s fine

  pair C; numeric R;
  C := (.5w, .5*(h - d));            % circle center
  R := .5*(w - 2u);                  % radius ( = .5*(h+d+2o) by construction)

  % outer circle (thicker pen, like 'pencircle scaled 1.5rule_thickness')
  pickup pencircle scaled (circle_factor*rule_thickness);
  draw fullcircle scaled (2R) shifted C;

  % --- inner strokes (round pen, matching the other line symbols) ---
  pickup pencircle scaled rule_thickness;

  % MF relations:
  % lft x9 = good.x(.5w - 3u); x9 = x10; x11 = x12 = w - x9
  x9  := .5w - 3u;  x10 := x9;  x11 := w - x9;  x12 := x11;

  % y10 = y12 = .5h - 6u; y9 = .5h + 2u; y11 = y9
  y10 := .5h - 6u;  y12 := y10;
  y9  := .5h + 2u;  y11 := y9;

  z9  = (x9,  y9);
  z10 = (x10, y10);
  z11 = (x11, y11);
  z12 = (x12, y12);

  % z13 = 1/3[z10,z9]; z14 = whatever[z11,z13] = whatever[z9,z12]
  z13 = 1/3[z10, z9];
  z14 = whatever[z11, z13] = whatever[z9,  z12];

  % draw segments
  draw z9--z10;      % left short vertical
  draw z13--z11;     % upper right diagonal
  draw z14--z12;     % lower right diagonal

  % match beginchar bbox
  fitbounds((0,-d - o)--(w,-d - o)--(w,h + o)--(0,h + o)--cycle);

  picture KOBIG; KOBIG = currentpicture;

endfig;

beginfig(52);

draw KO;

  % --- metrics (CM10-ish) ---
  u            := 10/18pt;          % ≈0.5556 pt
  w            := 18u;              % beginchar(...,18u,...)
  asc_height   := 6.94444pt;        % h
  desc_depth   := 2.0pt;            % d
  x_height     := 4.30556pt;        % ex
  rule_thickness := 0.40pt;

  % compute_spread(.2ex,.3ex) → take a mid value:
  spread := 0.25*x_height;

  h := asc_height;
  d := desc_depth;

  % MF beginchar had top = h + spread + rule_thickness
  o := spread + rule_thickness;     % makes top bounds = h + o

  % endpoints (rt x5 = w - 2.5u, lft x6 = 2.5u; ignore tiny ±eps)
  x6 := 2.5u; 
  x5 := w - 2.5u;

  % Centerline y so the top of the round stroke sits at h+o
  ybar := (h + o) - 0.5*rule_thickness;

  % draw the bar with rule-like pen
  pickup pencircle scaled rule_thickness;
  draw (x6, ybar) -- (x5, ybar);

  % bounds like beginchar(..., 18u, h+spread+rule_thickness, d)
  fitbounds((0,-d)--(w,-d)--(w,h+o)--(0,h+o)--cycle);

endfig;

beginfig(53);

draw KOBIG;

  % --- metrics (CM10-ish) ---
  u            := 10/18pt;          % ≈0.5556 pt
  w            := 18u;              % beginchar(...,18u,...)
  asc_height   := 6.94444pt;        % h
  desc_depth   := 2.0pt;            % d
  x_height     := 4.30556pt;        % ex
  rule_thickness := 0.40pt;

  % compute_spread(.2ex,.3ex) → take a mid value:
  spread := 0.25*x_height;

  h := asc_height;
  d := desc_depth;

  % MF beginchar had top = h + spread + rule_thickness
  o := spread + rule_thickness;     % makes top bounds = h + o

  % endpoints (rt x5 = w - 2.5u, lft x6 = 2.5u; ignore tiny ±eps)
  x6 := 2.5u; 
  x5 := w - 2.5u;

  % Centerline y so the top of the round stroke sits at h+o
  ybar := (h + o) - 0.5*rule_thickness;

  % draw the bar with rule-like pen
  pickup pencircle scaled rule_thickness;
  draw (x6, ybar) -- (x5, ybar);

  % bounds like beginchar(..., 18u, h+spread+rule_thickness, d)
  fitbounds((0,-d)--(w,-d)--(w,h+o)--(0,h+o)--cycle);

endfig;

beginfig(70);

  % CM-ish metrics
  u := 10/18pt;
  h := 6.94444pt;   % ascender
  d := 2.0pt;       % descender
  w := h + d;       % like beginchar(width=h+d)

  % outline style
  rule_thickness := 0.45pt;
  pickup pencircle scaled rule_thickness;

  % geometry
  pair C, vo, vi;
  numeric R, r, rotdeg;
  C := (0.5w, 0.5*(h - d));
  R := 0.5*(h + d);             % outer radius (hits y=h and y=-d)
  r := R*0.381966;              % inner radius = R/φ^2 (regular star)
  rotdeg := 0;                  % extra rotation if desired

  % start with "up" vectors and rotate by multiples of 72° (outer) / 36° offset (inner)
  vo := (0, R) rotated rotdeg;          % first outer vertex (top)
  vi := (0, r) rotated (rotdeg - 36);   % first inner vertex

  % build vertices z0..z9 alternating outer/inner
  z0 = C + vo;  z1 = C + vi;
  for i = 1 upto 4:
    vo := vo rotated -72;       z[2*i]   = C + vo;   % next outer
    vi := vi rotated -72;       z[2*i+1] = C + vi;   % next inner
  endfor

  % draw outline (hollow star)
  draw z0--z1--z2--z3--z4--z5--z6--z7--z8--z9--cycle;

  % explicit bounds (like beginchar)
  fitbounds((0,-d)--(w,-d)--(w,h)--(0,h)--cycle);

endfig;

% Minor circular arc through exact endpoints A and B about center C.
% Use cubic pieces spanning at most 45 degrees. Unlike angle-to-path-time
% conversion, this preserves both endpoints and their circular tangents.
vardef minorcirclearc(expr C, A, B) =
  save sweep, count, arcstep, k, P, Q, arc;
  numeric sweep, count, arcstep, k;
  pair P, Q; path arc;
  sweep := angle(B-C) - angle(A-C);
  if sweep > 180: sweep := sweep - 360 fi;
  if sweep < -180: sweep := sweep + 360 fi;
  count := max(1, ceiling(abs(sweep)/45));
  arcstep := sweep/count;
  k := (4/3)*sind(arcstep/4)/cosd(arcstep/4);
  P := A; arc := A;
  for i = 1 upto count:
    Q := if i = count: B else: C + (A-C) rotated (i*arcstep) fi;
    arc := arc .. controls (P + k*((P-C) rotated 90))
                      and (Q - k*((Q-C) rotated 90)) .. Q;
    P := Q;
  endfor
  arc
enddef;

beginfig(80);

  % metrics
  u := 10/18pt;
  w := 18u;
  h := 6.94444pt;  d := 2.0pt;
  rule_thickness := 0.40pt;

  % outer circle touching x=u, x=w-u and y=h+o, y=-d-o
  numeric o; o := 0.5*((w - 2u) - (h + d));
  pair   Co;  numeric Ro, s;
  Co = (0.5w, 0.5*(h - d));
  Ro = 0.5*(w - 2u);
  s  = Ro/sqrt(2);

  % circle points (do NOT declare z1..z8)
  z8 = Co + (0,  Ro);   z4 = Co + (0, -Ro);
  z6 = Co + (-Ro, 0);   z2 = Co + ( Ro,  0);
  z7 = Co + (-s,  s);   z1 = Co + ( s,  s);
  z3 = Co + ( s, -s);   z5 = Co + (-s,-s);

  % inner circle center on perp. bisector of chord z4--z7
  numeric bulge; bulge := 0.24;                 % 0.20 thin … 0.30 fuller
  pair Ci;
  Ci = 0.5[z4,z7] + bulge*unitvector((z7 - z4) rotated 90);

  % Outer circle; the inner arc is constructed through its exact endpoints.
  path circO, innerarc;
  circO := fullcircle scaled (2*Ro) shifted Co;

  % stroke style
  pickup pencircle scaled rule_thickness;

  % ---------- OUTER: clockwise arc z7 -> z4 via top/right ----------
  % fullcircle has exact eighth-circle knots: z7 at 3 and z4 at 6.
  draw reverse subpath (6, 11) of circO;

  % ---------- INNER: minor arc from z4 to z7 ----------
  innerarc := minorcirclearc(Ci, z4, z7);
  draw innerarc;

  % bounds like MF beginchar with overshoot
  fitbounds((0,-d - o)--(w,-d - o)--(w,h + o)--(0,h + o)--cycle);

endfig;

beginfig(81);

  % --- metrics (CM-ish) ---
  u := 10/18pt;                 % ≈ 0.5556 pt
  w := 18u;                     % beginchar(..., 18u, ...)
  h := 6.94444pt;               % asc_height
  d := 2.0pt;                   % desc_depth
  rule_thickness := 0.40pt;     % stroke weight (like rule.nib)
  eps := 0.01pt;                % tiny fudge like MF’s eps

  % Make the “circle_points” circle truly circular (MF’s overshoot trick)
  numeric o; o := 0.5*((w - 2u) - (h + d));
  pair   C;  numeric R;
  C = (0.5w, 0.5*(h - d));      % center = .5[z4,z8]
  R = 0.5*(w - 2u);             % radius (touches x=u and x=w-u)

  % --- outer circle with a uniform round stroke ---
  linecap := butt; linejoin := mitered;
  pickup pencircle scaled rule_thickness;       % uniform circular ring; a square pen distorts it
  draw fullcircle scaled (2R) shifted C;

  % --- central dot ---
  % MF: fill fullcircle scaled (length(z4-z8)/2 + eps) shifted (.5[z4,z8])
  % Here: |z4-z8| = 2R, so diameter = R + eps.
  fill fullcircle scaled (R + eps) shifted C;

  % bounding box like beginchar(..., 18u, h, d) with overshoot
  fitbounds((0,-d - o)--(w,-d - o)--(w,h + o)--(0,h + o)--cycle);

endfig;

end.