simpson's rule area and volume integrals using buildq and define

On Dec. 7, 2011, I wrote
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>Thanks to the examples of Robert Dodier and
>Barton Willis, I have written Simpson's one
>third rule code for area and volume integrals
>as a first step in improving the software  associated
>with ch. 8, Maxima by Example: Numerical Integration.
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Looking again at the volume code, I realize that
although it seems to work, I don't understand one
aspect.

The symbols %%u,  %%v, %%w should be bound
to the names of the three variables in the
furnished expression, and the %%v integral integrand is

G(%%u,%%v) = 
   integrate(ex(%%u,%%v,%%w),%%w,w1(%%u,%%v),w2(%%u,%%v)),

and the define statement uses dummy variables (us,vs) to pick out
a particular value of this quantity.

By mistake, I used the syntax
  subst([u=us,v=vs] ,ex)

instead of
  subst([%%u=us, %%v=vs],ex)

and the mistaken code seems to work.
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first the code version that doesn't work
(why not??)

(%i2) volume(ex,uL,vL,wL,nL) :=
 block([%%u,u1,u2,%%v,v1,v2,%%w,w1,w2,
              nu,nv,nw,u0,us,vs,hh,gg],
   local(hh,gg),
   [%%u,u1,u2] : uL,
   [%%v,v1,v2] : vL,
   [%%w,w1,w2] : wL,
   [nu,nv,nw] : nL,
   define(gg(us,vs),
        buildq([%%expr:subst([%%u=us,%%v=vs],ex),
                 %w1:subst([%%u=us,%%v=vs],w1),
                 %w2:subst([%%u=us,%%v=vs],w2) ],
         simp1(%%expr,%%w,%w1,%w2,nw))),
   define(hh(u0),
         buildq([%gg:subst(u0,%%u,gg(u,v)),
                  %v1:subst(u0,%%u,v1),
                  %v2:subst(u0,%%u,v2)],
            simp1(%gg,%%v,%v1,%v2,nv))),
   simp1(hh(%uu),%uu,u1,u2,nu))$

(%i3) volume(x*y^2*z^3,[x,0,1],[y,0,1],[z,0,1],[4,4,4]);

(%o3) 0.25*u*v^2
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and the not understood version that works:

(%i4) volume(ex,uL,vL,wL,nL) :=
 block([%%u,u1,u2,%%v,v1,v2,%%w,w1,w2,
              nu,nv,nw,u0,us,vs,hh,gg],
   local(hh,gg),
   [%%u,u1,u2] : uL,
   [%%v,v1,v2] : vL,
   [%%w,w1,w2] : wL,
   [nu,nv,nw] : nL,
   display(%%u,%%v,%%w),
   define(gg(us,vs),
        buildq([%%expr:subst([u=us,v=vs],ex),
                 %w1:subst([u=us,v=vs],w1),
                 %w2:subst([u=us,v=vs],w2) ],
         simp1(%%expr,%%w,%w1,%w2,nw))),
   define(hh(u0),
         buildq([%gg:subst(u0,%%u,gg(u,v)),
                  %v1:subst(u0,%%u,v1),
                  %v2:subst(u0,%%u,v2)],
            simp1(%gg,%%v,%v1,%v2,nv))),
   simp1(hh(%uu),%uu,u1,u2,nu))$

(%i5) volume(x*y^2*z^3,[x,0,1],[y,0,1],[z,0,1],[4,4,4]);

%%u = x

%%v = y

%%w = z

(%o5) 0.041666666666667
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Ted Woollett