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Big operators (metafontical answer)

It is a design feature of the displaystyle operators, that their boxen 
extend a bit over the baseline. The ``picture'' does not extend over the 
baseline (except for possibly one pixel because of rounding). I don't know 
the rationale behind this decision.

Here's a snippet out of bigop.mf to illustrate the situation, the relvant 
macro is ``padded'' which enlarges the height and depth by the amount given 
in sharp units.

cmchar "\textstyle set union sign";
beginchar(oct"123",15u#,0,10/6dh#);
adjust_fit(0,0); pickup pencircle scaled stem;
lft x1=hround u; x2=x1; x3=w-x3; x4=x5=w-x1;
top y1=eps; bot y3=-d; y2=y4=2/3[y1,y3]; y5=y1;
draw z1---z2...z3...z4---z5;  % stems and cup
labels(1,2,3,4,5); endchar;

cmchar "\displaystyle set union sign";
beginchar(oct"133",20u#,0,14/6dh#); padded 1/6dh#;
adjust_fit(0,0); pickup pencircle scaled curve;
lft x1=hround u; x2=x1; x3=w-x3; x4=x5=w-x1;
top y1=eps; bot y3=-d; y2=y4=2/3[y1,y3]; y5=y1;
draw z1---z2...z3...z4---z5;  % stems and cup
labels(1,2,3,4,5); endchar;

--J"org Knappen