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Mediocre pop count

Given a sequence of letters, omit those with the highest or lowest pop count.

## Terminology
The pop count or [population count] of a binary string is the number of `1`s in it.

For this challenge, the pop count of a letter is the number of `1`s in its ASCII character code in binary.

For example, the letter "Z" has ASCII character code 90, which is `1011010` in binary. There are 4 `1`s in this binary string, so the pop count of "Z" is 4.

Here is the pop count of every letter, in the format `letter : pop count`:

```text
A : 2
B : 2
C : 3
D : 2
E : 3
F : 3
G : 4
H : 2
I : 3
J : 3
K : 4
L : 3
M : 4
N : 4
O : 5
P : 2
Q : 3
R : 3
S : 4
T : 3
U : 4
V : 4
W : 5
X : 3
Y : 4
Z : 4
a : 3
b : 3
c : 4
d : 3
e : 4
f : 4
g : 5
h : 3
i : 4
j : 4
k : 5
l : 4
m : 5
n : 5
o : 6
p : 3
q : 4
r : 4
s : 5
t : 4
u : 5
v : 5
w : 6
x : 4
y : 5
z : 5
```
## Input
- A sequence of letters
- This may be a string or any ordered data structure of characters
- The letters may contain a mixture of upper and lower case
- The characters that count as letters for this challenge are A-Z and a-z, specifically `ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz`
- The input may be empty (it may have length zero)

## Output
- A sequence of letters
- This may be a string or any ordered data structure of characters. It does not need to match the input format (provided it is consistent between inputs)
  - For example, you may take input as an array of characters, and output as a string, provided this does not change for different inputs
- All of the letters from the input are included in the output in the same order, except for those with the highest or lowest pop count, which are omitted
- If all of the letters have either the highest or lowest pop count, the output is empty (a sequence of length zero)
- Case must be maintained
  - a letter that is lower case in the input must be lower case in the output (if present)
  - a letter that is upper case in the input must be upper case in the output (if present)

## Test cases
Test cases are in the format `"input" : "output"`

```text
"" : ""
"a" : ""
"ab" : ""
"abc" : ""
"abcd" : ""
"abcde" : ""
"abcdef" : ""
"abcdefg" : "cef"
"A" : ""
"AB" : ""
"ABC" : ""
"ABCD" : ""
"ABCDE" : ""
"ABCDEF" : ""
"ABCDEFG" : "CEF"
"oAPwSoeHcoDretMBBoesoBsagHDoew" : "SecretMessage"
"SecretMessage" : "SecretMee"
```

> Explanations are optional, but I'm more likely to upvote answers that have one.



[population count]: https://en.wikipedia.org/wiki/Hamming_weight "Wikipedia page for 'Hamming weight' - a generalisation of pop count"