1; 2; 3; 4; 5 // 1 кусок
6; 5; 4; 3    // 2 кусок
              // 3 кусок пустой
-3; -3; -3    // 4 кусок

1; 2; 3; 4; 5;   5.5;    6;  5;  4;  3;      0;      -3; -3; -3
// 1-ая часть  (5 + 6)/2  2-ая часть     (3 + -3)/2    4-я часть

IEnumerable<double> FlattenWithInsertedValues(IEnumerable<IEnumerable<double>> seqs)
{
    double? firstAddend = null;
    foreach (var seq in seqs)
    {
        double? lastInThisSeq = null;
        foreach (var v in seq)
        {
            if (firstAddend != null)
            {
                yield return (firstAddend.Value + v) / 2;
                firstAddend = null;
            }
            yield return v;
            lastInThisSeq = v;
        }
        if (lastInThisSeq != null)
            firstAddend = lastInThisSeq;
    }
}

void Test1()
{
    var result = FlattenWithInsertedValues(TestGenerator1());
    foreach (var v in result)
        Console.Write(v + " ");
}

void Test2()
{
    var result = FlattenWithInsertedValues(TestGenerator2());
    Console.Write(result.Count());
}

IEnumerable<IEnumerable<double>> TestGenerator1()
{
    Console.Write("Generating outer seq 1 ");
    yield return Generator(1, 5, 1);
    yield return Generator(1, 0, 2);
    yield return Generator(1, 3, 3);
}

IEnumerable<IEnumerable<double>> TestGenerator2()
{
    Console.Write("Generating outer seq 2 ");
    yield return Generator(1, 5, 1);
    yield return Generator(1, 10000000000000, 2);
}

IEnumerable<double> Generator(double from, double to, int seqNum)
{
    Console.Write($"Generating seq #{seqNum} ");
    for (double curr = from; curr < to; curr += 1)
        yield return curr;
}

def flatten(*lists):
  prev = None
  for lst in lists:
    for elt in lst:
      if prev is not None:
         yield (prev + elt) / 2
         prev = None
      yield elt
    prev = elt

lst = [[1, 2, 3, 4, 5], [6, 5, 4, 3], [-3,  -3, -3]]
print(list(flatten(*lst)))

let flatten seqs =
    // нумеруем каждый элемент внутренней пос-ти его индексом
    let numbering = Seq.zip (Seq.initInfinite id) in
    // сливаем внутренние посл-ти
    let pairs = (Seq.map numbering >> Seq.concat) seqs in

    let folder (prev_i, prev_elem, _) (cur_i, cur_elem) =
        // если индексы текущего и предыдущего элементов не последовательны
        // значит, это граница, добавляем средний
        let is_bound = cur_i - prev_i <> 1 in
        let with_mean =
            if is_bound then [| (cur_elem + prev_elem) / 2.; cur_elem |]
            else [| cur_elem |] in
        (cur_i, cur_elem, Seq.ofArray with_mean) in

    let take_3rd = function | _, _, x -> x in
        (Seq.scan folder (-1, 0., Seq.empty)
            >> Seq.skip 1
            >> Seq.map take_3rd
            >> Seq.concat) pairs

[[1; 2; 3; 4; 5]; [6; 5; 4; 3]; []; [-3; -3; -3]]
    |> List.map (List.map float) |> List.map Seq.ofList |> Seq.ofList
    |> flatten
    |> List.ofSeq
    |> printfn "%A"

seq {
    printfn "Generating outer seq";
    yield seq { printfn "Generating inner seq 1"; yield 1.0; };
    yield seq { printfn "Generating inner seq 2"; yield 2.0; };
    yield seq { printfn "Generating inner seq 3"; yield 3.0; }
}
|> flatten
|> List.ofSeq
|> printfn "%A"

[1.0; 2.0; 3.0; 4.0; 5.0; 5.5; 6.0; 5.0; 4.0; 3.0; 0.0; -3.0; -3.0; -3.0]
Generating outer seq
Generating inner seq 1
Generating inner seq 2
Generating inner seq 3
[1.0; 1.5; 2.0; 2.5; 3.0]

FLOAT next(){
static FLOAT prev = NAN;
static bool isFirst = false;
static Seq<float> local = GLOBAL.next();
if (!local)
    return null; //end result seq
if (isFirst)     //it's first elem in seq so we read it before
    isFirst = false;
    return prev;
}
FLOAT temp = local.next();
if (!temp){ //if we read current seq
    local = GLOBAL.next();
    if (!local)      //we read all seq
        return null; //end result seq
    FLOAT tmp = prev;
    prev = local.next();
    if (!prev){      //if empty seq
        prev = NAN;
        local = GLOBAL.next(); //go next seq
        return NAN;
    }
    isFirst = true; // we read first elem need save it
    return (prev + tmp)/2;
}
return prev = temp;
}

flatten :: [[Float]] -> [Float]
flatten [] = []
flatten [y] = y
flatten ([]:xs) = flatten xs
flatten (y:[]:xs) = flatten $ y:xs
flatten ([y]:(yy:ys):xs) = y : (y + yy)/2 : flatten ((yy:ys):xs)
flatten ((y:ys):zs:xs) = y : flatten (ys:zs:xs)

main = do
  let ls = [[1, 2, 3, 4, 5], [6, 5, 4, 3], [-3,  -3, -3]]
  print ls
  let fs = flatten ls
  print fs

#include <math.h>
#include <unistd.h>
#include <sysexits.h>

#define DBLRead(fd, d)  (read(fd, d, sizeof(double)) == sizeof(double))
#define DBLWrite(fd, d) (write(fd, d, sizeof(double)) == sizeof(double))

int
flatten (int n, int in[], int out)
{
  int i, j, rc = 0;
  double d0 = NAN, d;

  for (i = 0; !rc && i < n; close(in[i++]))
    for (j = 0; !rc && DBLRead(in[i], &d); j++) {
      if (!isnan(d0) && j == 0) {
        double m = (d0 + d) / 2;
        if (!DBLWrite(out, &m)) {
          rc = EX_IOERR;
          break;
        }
      }
      if (!DBLWrite(out, &d))
        rc = EX_IOERR;
      d0 = d;
    }


  close(out);
  return rc;
}

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <err.h>
#include <sysexits.h>
#include <sys/types.h>
#include <sys/wait.h>

int
generate (int p[2], double from, int step, int n)
{
  if (fork()) {
    close(p[1]);
    return p[0];
  }

  int i, rc = 0;

  for (i = 0; i < n; i++, from += step)
    if (!DBLWrite(p[1], &from)) {
      rc = EX_IOERR;
      break;
    }

  exit(rc);
}

#define A_SIZE(a) (sizeof(a) / sizeof(a[0]))

int
main (int ac, char *av[])
{
  int p1[2], p2[2], p3[2], p4[2], res[2];
  if (pipe(p1) || pipe(p2) || pipe(p3) || pipe(p4) || pipe(res))
    err(EX_OSERR, "pipes");
  int gen_input[4] = {
    generate(p1, 1.0, 1, 5),
    generate(p2, 6.0, -1, 4),
    generate(p3, 1.0, 1, 0),
    generate(p4, -3.0, 0, 3)
  };

  if (!fork())
    exit(flatten(A_SIZE(gen_input), gen_input, res[1]));
  close(res[1]);

  double r = 0;
  while(DBLRead(res[0], &r) || (puts(""), 0))
    printf("%g ", r);

  pid_t pid;
  int s;
  while ((pid = wait(&s)) > 0)
    if (!WIFEXITED(s) || WEXITSTATUS(s))
      printf("pid: %d rc = %dn", (int)pid, WEXITSTATUS(s));

  return puts("End") == EOF;
}

def inter_means(sequences)
  Enumerator.new do |y|
    last_inserted = nil
    sequences.lazy.each do |seq|
      first = true
      seq.lazy.each do |element|
        if first && last_inserted
          y << (element + last_inserted) / 2.0
          last_inserted = nil
        end
        first = false
        y << element
        last_inserted = element
      end
    end
  end
end

inputs = [
  [[*1..5], [*3..6].reverse, [], [-3, -3, -3]],
  [],
  [[], [], []],
  [[], [1, 2, 3], [], [-3, -2, -1], [], []]
]
inputs.each do |input|
  puts "  #{input.inspect}n=> #{inter_means(input).to_a.inspect}"
end

[[1, 2, 3, 4, 5], [6, 5, 4, 3], [], [-3, -3, -3]]
=> [1, 2, 3, 4, 5, 5.5, 6, 5, 4, 3, 0.0, -3, -3, -3]
  []
=> []
  [[], [], []]
=> []
  [[], [1, 2, 3], [], [-3, -2, -1], [], []]
=> [1, 2, 3, 0.0, -3, -2, -1]

(inter-means [])
(inter-means [[1 2 3 4 5] [6 5 4 3] [] [-3 -3 -3]])