folds

   what               k4/k7         k9              example    result                
  ---------------------------------------------------------------------------------
   each-left         a f\:b     a f\:b            (!3)*\:!2    (0 0;0 1;0 2)         
   each-right        a f/:b     a f/:b            (!3)*/:!2    (0 0 0;0 1 2)         
   fold                 f/v        f/v                */6 7    42                    
   fold w/initial     a f/v      a f/v             7*/11 13    1001                  
   scan                 f\v        f\v              -\1 1 1    1 0 -1                
   scan w/initial     a f\v      a f\v             3-\1 1 1    2 1 0                 
   fixpoint             f/x       f/:x            (1+1%)/:1    1.618034              
   fixpoint scan        f\x       f\:x           3 4 2 1\:0    0 3 1 4               
   do-n               n f/x   (n;f)/:x       (2;"ha",)/:"!"    "haha!"               
   do-n scan          n f\x   (n;f)\:x         (3;{x*x})\:2    2 4 16 256            
   while              c f/x   (c;f)/:x         (1e3>;2*)/:1    1024                  
   while scan         c f\x   (c;f)\:x        (5 mod;2+)\:4    4 6 8 10              

   e.g. flatten         ,//       ,//:      ,//:((1;2 3);4)    1 2 3 4               

   sv                  b/:v        b/v              16/2 10    42                    
   vs                  b\:x        b\x                 2\42    1 0 1 0 1 0           

   i:&'3={y*1+x}\'" "=b:2_(#')#0:"folds.txt";&/(~/$.')'(i^'b)[;3 4]

flatten could use a better example. if you do k9 ,// the result is the same (no fixed-point involved).
how about: ,//:((1;2 3);4)

@ktye yes! thanks

mi 2021.09.20 2 16 (c)shakti 2.0
 (3;{x*x})\:2
OS
!nyi
 (1e3>;2*)/:1
!type
 (5 mod;2+)\:4
OS
!nyi