Manipulating matrices for 3D drawing I've written a short library for manipulating matrices for 3D drawing. I've tried to strike a balance between speed and readability. Anything to improve?

%!
%mat.ps
%Matrix and Vector math routines

/ident { 1 dict begin /n exch def
    [
    1 1 n { % [ i
        [ exch % [ [ i
        1 1 n { % [ [ i j
            1 index eq { 1 }{ 0 } ifelse % [ [ i b
            exch % [ [ b i
        } for % [ [ b+ i
        pop ] % [ [ b+ ]
    } for % [ [b+]+ ]
    ]
end } def

/rotx { 1 dict begin /t exch def
    [ [ 1  0      0         ]
      [ 0  t cos  t sin neg ]
      [ 0  t sin  t cos     ] ]
end } def

/roty { 1 dict begin /t exch def
    [ [ t cos      0  t sin ]
      [ 0          1  0     ]
      [ t sin neg  0  t cos ] ]
end } def

/rotz { 1 dict begin /t exch def
    [ [ t cos  t sin neg  0 ]
      [ t sin  t cos      0 ]
      [ 0      0          1 ] ]
end } def

/.error where { pop /signalerror { .error } def } if

/dot { % u v
    2 copy length exch length ne {
        /dot cvx /undefinedresult signalerror } if
    % u v
    0 % u v sum
    0 1 3 index length 1 sub { % u v sum i
        3 index 1 index get exch % u v sum u_i i
        3 index exch get % u v sum u_i v_i
        mul add % u v sum
    } for % u v sum 
    3 1 roll pop pop % sum
} bind def

% [ x1 x2 x3 ] [ y1 y2 y3 ]  cross  [ x2*y3-y2*x3 x3*y1-x1*y3 x1*y2-x2*y1 ]
/cross { % u v
    dup length 3 ne 2 index length 3 ne or {
        /cross cvx /undefinedresult signalerror } if
    % u v
    exch aload pop 4 3 roll aload pop % x1 x2 x3 y1 y2 y3
    [
        5 index 2 index mul % ... [ x2*y3
        3 index 6 index mul sub % ... [ x2*y3-y2*x3
        5 index 5 index mul % ... [ x2*y3-y2*x3 x3*y1
        8 index 4 index mul sub % ... [ x2*y3-y2*x3 x3*y1-x1*y3
        8 index 5 index mul % ... [ x2*y3-y2*x3 x3*y1-x1*y3 x1*y2
        8 index 7 index mul sub % ... [ x2*y3-y2*x3 x3*y1-x1*y3 x1*y2-x2*y1
    ]
    7 1 roll 6 { pop } repeat
} bind def

/transpose { STATICDICT begin
    /A exch def
    /M A length def
    /N A 0 get length def
    [
    0 1 N 1 sub { /n exch def
        [
        0 1 M 1 sub { /m exch def
            A m get n get
        } for
        ]
    } for
    ]
end } dup 0 6 dict put def

/matmul { STATICDICT begin
    /B exch def
    B 0 get type /arraytype ne { /B [B] def } if
    /A exch def
    A 0 get type /arraytype ne { /A [A] def } if
    /Q B length def
    /R B 0 get length def
    /P A length def
    Q A 0 get length ne {
        /A A transpose def
        /P A length def
        Q A 0 get length ne {
            A B end /matmul cvx /undefinedresult signalerror
        } if
    } if

    [
    0 1 P 1 sub { /p exch def % rows of A
        [
        0 1 R 1 sub { /r exch def % cols of B
            0
            0 1 Q 1 sub { /q exch def % terms of sum
                A p get q get
                B q get r get mul
                add
            } for
        } for
        ]
    } for
    ]

end } dup 0 10 dict put def

%u v {operator}  vop  u(op)v
%apply a binary operator to corresponding elements
%in two vectors producing a third vector as result
/vop { 1 dict begin
    /op exch def
    2 copy length exch length ne {
        /vop cvx end /undefinedresult signalerror
    } if

    [ 3 1 roll % [ u v
    0 1 2 index length 1 sub { % [ ... u v i
        3 copy exch pop get % u v i u_i
        3 copy pop get      % u v i u_i v_i
        op exch pop         % u v u_i(op)v_i
        3 1 roll            % u_i(op)v_i u v
    } for % [ ... u v
    pop pop ]

end } def

%length of a vector
/mag { 0 exch { dup mul add } forall } def


/elem { % M i j
    3 1 roll get exch get % M_i_j
} def



/det {
    dup length 1 index 0 get length ne { /det cvx /typecheck signalerror } if
    1 dict begin /M exch def
        M length 2 eq {
            M 0 0 elem
            M 1 1 elem mul
            M 0 1 elem
            M 1 0 elem mul sub
        }{
            M length 3 eq {
                M aload pop cross dot
            }{ /det cvx /rangecheck signalerror } ifelse
        } ifelse
    end
} def
As far as readability goes, maybe you'd like to add an arg1 arg2 arg3 -- result1 result2 type of comment to each function? So we know what was on the stack before invoking, and what we expect to find afterward? Almost equivalently, you might show a unit test invoking your function and verifying the result.