section 6C
X_lens_permu_form

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Classes var_lens_permu_form and con_lens_permu_form appear to permute the blts residing within the form of the underlying matrix; subscripts must be correspondingly permuted. This is a generalization of the notion of transposing a matrix. For example, this program:

```#include "SOME_DIRECTORY/mat_gen_dim.h"
using namespace mat_gen_dim;

int main () {
permu const per {1, 2, 0};

form const frm_a {blt{2,5},blt{1,3},blt{4,8}};
cout << "\nfrm_a: " << frm_a;
var_matrix<double> mat_a {sub_matrix_linear (frm_a, 6.0)};
cout << "\nmat_a: " << mat_a;

var_lens_permu_form<double> aof_b {mat_a, per};
cout << "\nfrm_b: " << aof_b.get_form();
cout << "\naof_b: " << aof_b;

con_lens_permu_form<double> aof_c {aof_b, per};
cout << "\nfrm_c: " << aof_c.get_form();
cout << "\naof_c: " << aof_c;

cout << "\nmat_a (3 1 6) = " << mat_a.get (vec_si {3,1,6});
cout << "\naof_b (1 6 3) = " << aof_b.get (vec_si {1,6,3});
cout << "\naof_c (6 3 1) = " << aof_c.get (vec_si {6,3,1});

return 0;
}
```
gives this output:
```frm_a: ( 2 =< 5, 1 =< 3, 4 =< 8 )
mat_a:
( 2 1 4 ) = 6.214    ( 2 1 5 ) = 6.215    ( 2 1 6 ) = 6.216    ( 2 1 7 ) = 6.217
( 2 2 4 ) = 6.224    ( 2 2 5 ) = 6.225    ( 2 2 6 ) = 6.226    ( 2 2 7 ) = 6.227
( 3 1 4 ) = 6.314    ( 3 1 5 ) = 6.315    ( 3 1 6 ) = 6.316    ( 3 1 7 ) = 6.317
( 3 2 4 ) = 6.324    ( 3 2 5 ) = 6.325    ( 3 2 6 ) = 6.326    ( 3 2 7 ) = 6.327
( 4 1 4 ) = 6.414    ( 4 1 5 ) = 6.415    ( 4 1 6 ) = 6.416    ( 4 1 7 ) = 6.417
( 4 2 4 ) = 6.424    ( 4 2 5 ) = 6.425    ( 4 2 6 ) = 6.426    ( 4 2 7 ) = 6.427
frm_b: ( 1 =< 3, 4 =< 8, 2 =< 5 )
aof_b:
( 1 4 2 ) = 6.214    ( 1 4 3 ) = 6.314    ( 1 4 4 ) = 6.414
( 1 5 2 ) = 6.215    ( 1 5 3 ) = 6.315    ( 1 5 4 ) = 6.415
( 1 6 2 ) = 6.216    ( 1 6 3 ) = 6.316    ( 1 6 4 ) = 6.416
( 1 7 2 ) = 6.217    ( 1 7 3 ) = 6.317    ( 1 7 4 ) = 6.417
( 2 4 2 ) = 6.224    ( 2 4 3 ) = 6.324    ( 2 4 4 ) = 6.424
( 2 5 2 ) = 6.225    ( 2 5 3 ) = 6.325    ( 2 5 4 ) = 6.425
( 2 6 2 ) = 6.226    ( 2 6 3 ) = 6.326    ( 2 6 4 ) = 6.426
( 2 7 2 ) = 6.227    ( 2 7 3 ) = 6.327    ( 2 7 4 ) = 6.427
frm_c: ( 4 =< 8, 2 =< 5, 1 =< 3 )
aof_c:
( 4 2 1 ) = 6.214    ( 4 2 2 ) = 6.224    ( 4 3 1 ) = 6.314    ( 4 3 2 ) = 6.324
( 4 4 1 ) = 6.414    ( 4 4 2 ) = 6.424    ( 5 2 1 ) = 6.215    ( 5 2 2 ) = 6.225
( 5 3 1 ) = 6.315    ( 5 3 2 ) = 6.325    ( 5 4 1 ) = 6.415    ( 5 4 2 ) = 6.425
( 6 2 1 ) = 6.216    ( 6 2 2 ) = 6.226    ( 6 3 1 ) = 6.316    ( 6 3 2 ) = 6.326
( 6 4 1 ) = 6.416    ( 6 4 2 ) = 6.426    ( 7 2 1 ) = 6.217    ( 7 2 2 ) = 6.227
( 7 3 1 ) = 6.317    ( 7 3 2 ) = 6.327    ( 7 4 1 ) = 6.417    ( 7 4 2 ) = 6.427
mat_a (3 1 6) = 6.316
aof_b (1 6 3) = 6.316
aof_c (6 3 1) = 6.316
```
In this example, the same permutation is applied twice, giving a different result each time because the effect accumulates. By contrast, the effect of X_lens_other_form does not accumulate.