[Question]
  [
Write a program/function that takes two integers in the range \$0\$ to \$255\$ inclusive, and returns whether the binary forms of the numbers are exactly one bit different. 
For example, \$1\$ and \$0\$ have binary forms `00000001` and `00000000`, which are one bit apart. Similarly, \$152\$ and \$24\$ are `010011000` and `000011000`, so they return true.
***However***, your code must be **pristine**, such that if any one bit in your program is flipped, it should throw an error. For example, if your program was the single byte `a` (`01100001`), then all the 8 possible modified programs:
```
á ! A q i e c `
```
must throw an error. Make sure you are modifying by *bytes* (e.g. the `á` up there is actually representing the byte \$225\$, not the actual two byte character `á`).
### Test cases:
```
0,1     => Truthy
1,0     => Truthy
152,24  => Truthy
10,10   => Falsey
10,11   => Truthy
11,12   => Falsey
255,0   => Falsey
```
## Rules:
* Provide a testing framework that can verify that your program is properly pristine, since there will be a lot of possible programs (number of bytes\*8), or else a complete proof of pristineness.
	+ *Please* make sure your program is valid before you post it.
* Output needs to be either truthy/falsey (either way around is fine), or else two distinct non-error values
* Errors can be runtime, compiler, interpreter etc.
  
[Answer]
# [Python 2](https://docs.python.org/2/), 35 bytes
```
lambda a,b:(a^b)&-(a^b)in[a^b or[]]
```
[Try it online!](https://tio.run/##fVHdbsIgGL3nKYgmAyya0szEkHSXewLvms7QFidLpYSypW7Zs3dgrc65jRv4zl8OYA5u1@ikL5tKppPJpK/FvqgEFLTgWDwV5G5@3JTO/AYbm@V573UZ43OWAyA7WWK0TVEUEggAxirt4BbHlBE4hWGlD3BtX93ucCYZjf8hlwlN7o/8L6TPPXk9@SjqVl6T7Ez@dDLKkr@cyXJJb2IHFoApXMvWqxoLPdI6pSUIt90USqcILV4apTH64PGq@EQLL9sLhxtb4ZKQYCqh0nB4njAWygXACv0scS01HrMI4QCOw2ZbK2NkBdMzlHFvzSMUM5SdsQClKYpRHn3HIsbzU9glaOxa7iz2N8Pj3BGaDFW70OxdGTzLlJMW37Qh@WxFCIDOHnxZeP39o8rzl3/05@GV0ZuoVcURtdKcki962ZXSOG5E2/Zf "Python 2 – Try It Online")
Uses the power-of-two check `n&-n==n`, eliminating the `n==0` false positive.
For reference, these are the pairs of one-char binary operators that are one bit apart, making them hard to use:
```
+ /
- /
* +
% -
< |
< >
```
Fortunately, `&` and `^` are not among these.
Also note that `==` can become `<=`, and `+` can become the comment character `#`.
---
# [Python 2](https://docs.python.org/2/), 41 bytes
```
lambda a,b:bin(a^b).count(`+True`)is+True
```
[Try it online!](https://tio.run/##fVFBbsIwELz7FRYcbBMXxVGRUKT02Bdwi1JwElNcBcdyTBVa9e2pTUiA0tYn78zsaHZXH@2uVlFX1KVIJpNJV/F9XnLIaR7nUmH@kpN5UR@UxZtgZQ5iQ2Rz@nROnbL4gWUAiFYUGG0TFHgfAoA2Ulm4xSFlBE6hf8kTdG12dxxJRsN/yEVEo8cT/wvpfM@9jnzmVSNuSTaSPzsZZdFfndFiQe9sexaAKVyJxqlqAx3SWKkE8NOu3ZoShOZvtVsX@ozDZf6F5k625xbXpsQFIb6pgFLBfj2@zKX1gOHqVeBKKDx4ERIDOBTrbSW1FiVMRih1d7FZgEKG0hHzUJKgEGXBNRawODubXYyGrMXOYDcZHuqW0KiP2vpkH1LjWSqtMPguDclmS0IAtObowsLb8w8qx1/u6P79ltE7r2QZI2qEPjtf9KIthLax5k3TfQM "Python 2 – Try It Online")
Taking [TFeld's](https://codegolf.stackexchange.com/a/182836/20260) `lambda a,b:bin(a^b).count('1')==1` and making it pristine by changing the 1's to `+True` and `==` to `is`. Thanks to Jo King for 1 byte.
[Answer]
# [Python 2](https://docs.python.org/2/), ~~72~~ ~~67~~ 50 bytes
```
lambda a,b:sum(map(int,'{:b}'.format(a^b)))is+True
```
[Try it online!](https://tio.run/##fVHRToMwFH3vVzTbQ9tRF0pcspDgo1@wN4JLgeJqoDSlM0zjt2Mrgzmn8sQ9556T03P1yR5aFQ1FW4pksVgMNW/ykkNO87g7NrjhGktlKXqP8w@0rlrTcIv5U04IkV2wM0cxOFnK4juWASB6UWBUJSjwhgQAbZwaVjikjMAl9F/yAJ3MHk4zyWj4D7mJaHT/xf9COt@z1pGPvO7ENclm8qeSURb9pYw2G3pjO7IALOFOdG6rNdAhnZVKAP/afS5VgtD6pZUKu77C7bfGWlPighAvKqBUcKzHj7m0HjBcPQtcC4UnL0JiAKdhX9VSa1HCZIbS2EmzAIUMpTPmoSRBIcqC71jA4uxsdjGashYH42@Mp7knNBqj9j7Zm9R4lUorDL5JQ7LVlhAArTm5sPD6/NOW4y93dP9jy@iV17KMETVCn50v@6IvhLax5l03fAI "Python 2 – Try It Online")
-5 bytes, thanks to Jo King
---
Returns `True`/`False` for for truthy/falsey. 
The program is basically the same as `lambda a,b:bin(a^b).count('1')==1`, but without numbers and other chars which work when bit-flipped.
Works by making sure that almost everything is a named function (which are all quite pristine)
The pristine test at the end flips a single bit (for every bit), and tries the function on an input. If that works (correct or not), that variation is printed. No printed programs = pristine function.
[Answer]
# Java 8, ~~68~~ ~~61~~ ~~56~~ 45 bytes
```
a->b->(a.bitCount(a^b)+"").equals(-~(a^a)+"")
```
-11 bytes thanks to *@EmbodimentOfIgnorance*, replacing [constant](https://docs.oracle.com/javase/8/docs/api/constant-values.html#org.w3c) `java.awt.Font.BOLD` with `-~(a^a)`.
[Try it online.](https://tio.run/##lY8xa8MwEIV3/4ojk0RrEZt6SuKhgUCGThlLC2dHNnIVybFOgVDSv@6qxU2hS5Xt8d538F2HJ0y7/dtYa3QOnlCZ9wTAEZKqoQur8KS0aLypSVkjNlNYbg3JVg73cdCjtVqiKUtoYDViWlZpyVBUitbWG2L4WvG72YwLefSoHUs/QoXf1bhIglHvKx2MJrGTVXs4BFm2o0GZ9vkFcGgd/3IH2J0dyYOwnkQfVtKGNQL7Xp/ZnE8h43zxH5zx61UEXOQ/eP4Qw/@qzG/Do9Sv7lkegedF8efXS3IZPwE)
**Explanation:**
The shortest base function would be:
```
a->b->a.bitCount(a^b)==1
```
[Try it online.](https://tio.run/##lY9Bi4MwEIXv/oo5GliDynpq9dCFhR568rhsYbQqsWkiZlKQpb/dTcG20EvT24P3De@bHs8Y9YfjXEs0BnYo1F8AYAhJ1NC7llsSkrdW1SS04t9LWG8VNV0zfvhBG61lg6oooIV8xqioogJ5JehLW0Uh7iuW58m8Ctz4YCvpxheHsxYHODmvsKRRqO7nF3DsDLtqApSToebEtSU@uJakCluOwyCnMGZLSBhbvYITdr/ygLP0hqefPvxDJX4P91K/uyepB55m2dOvl@Ay/wM)
This is modified so there isn't a digit, `=`, nor one of the `+/*` operands in it for numeric calculations (so the `+` for String-concatenation is fine):
The `+""` and `.equals` are to compare by `String.equals(String)` instead of `int==int`.  
NOTE: `Integer.equals(int)` could be used here, but would be more bytes, since both the `.bitCount` and `java.awt.Font.BOLD` are primitive `int` instead of `Integer`-objects, so an additional `new Integer(...)` would be required to transform one of the two to an `Integer`-object, before we could use the `.equals`.
[Answer]
# [C (gcc)](https://gcc.gnu.org/), 56 bytes
```
d(a,b){return(sizeof((char)d))^__builtin_popcount(a^b);}
```
[Try it online!](https://tio.run/##bZDBDoIwDIbvPEWPnWIiJnqZPIkRMsbQJTrI3Iwb4dlxSFCM9tC06ff/TctXJ877vkQWF6TVwlit8Ca9qCtEfmaalIRkeV5YeTFS5U3d8NoqgywrCO16qQxcmVQ4FEyfeAyDChahvh@OBNoIQgxTF3v6aqpaA3pIYU3Bwx422x2F5dJP8JtxI@M@jJszk7EWN3sxgS3Rx47QH8Aq8WgEN6IMEP45xmfBOE0hCeJvdQU42aczH/JFDTG@DpLP9i4aczQbr2nU9U8 "C (gcc) – Try It Online")
Returns `0` if the pair differ by 1, non-zero otherwise. Slightly unusual for C, unless you consider it returning `EXIT_SUCCESS` if the pair differ by 1, any other value otherwise.
Uses `sizeof((char)d))` to produce the constant `1` in a pristine way while also forcing the function name to be pristine.
It then XORs that 1 with the popcount of the XOR of the arguments. Luckily the `^` symbol is easy to keep pristine, as is the very long identifier `__builtin_popcount`. 
Meanwhile, here is the script used to test the solution:
```
#!/bin/bash
SOURCE_FILE=$1
FOOT_FILE=$2
TMP_SRC=temp.c
LENGTH="$(wc -c <"$SOURCE_FILE")"
BITS=$((LENGTH*8))
cat "$SOURCE_FILE" >"$TMP_SRC"
cat "$FOOT_FILE" >>"$TMP_SRC"
if gcc -w $TMP_SRC -o t.out >/dev/null 2>&1; then
    if ./t.out; then
        echo "Candidate solution..."
    else
        echo "Doesn't even work normally..."
        exit
    fi
else
    echo "Doesn't even compile..."
    exit
fi
for i in $(seq 1 $BITS); do
    ./flipbit "$i" <"$SOURCE_FILE" >"$TMP_SRC"
    cat "$FOOT_FILE" >>"$TMP_SRC"
    if gcc -w $TMP_SRC -o t.out >/dev/null 2>&1; then
        echo "Testing flipped bit $i:"
        cat "$TMP_SRC"
        ./t.out >/dev/null 2>&1
        STATUS=$?
        if [ "$STATUS" -eq 0 ]; then
            echo "It works!"
            exit
        elif [ "$STATUS" -eq 1 ]; then
            echo "It doesn't work..."
            exit
        else
            echo "It crashes"
        fi
    fi
done
```
Which uses the `./flipbit` tool I wrote whose source is simply:
```
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
    int bittoflip = atoi(argv[1]) - 1;
    int ch;
    while ((ch = fgetc(stdin)) != EOF) {
        if (bittoflip < 8 && bittoflip >= 0) {
            putchar(ch ^ (1 << bittoflip));
        } else {
            putchar(ch);
        }
        bittoflip -= 8;
    }
    return 0;
}
```
The tricky bits were:
* Whitespace: All whitespace (including newlines) have pristine twins that will work similarly
* Comparison: `=` doesn't work well, since it can be a comparison in every case it could appear. Similarly `-` doesn't work well. Thus `^` is used to assert equality with 1.
* Variable names: f would clash with b, so had to use d as the function name instead.
[Answer]
# [R](https://www.r-project.org/), ~~38~~ 37 bytes
-1 byte thanks to Nick Kennedy.
```
dpois(log2(bitwXor(scan(),scan())),T)
```
[Try it online!](https://tio.run/##VYtBCsMgFET3/xQu/dSKSqWbHiOLQsiiTbEIQUu01BBydqsGCl29YebNnA25HIl5uzFa7yjnHFdItROwVEw2xFZDGG/uT8a1iKVI5EAkLH2fhmHLj5e3gU7@qejdxs/Vz7Q@KbIdiKzDvIGhghGJhZIR0agVU6eW6iR@abeKJlVNSuv6yIUg1Rm@ "R – Try It Online") (Thanks to Giuseppe for setting up the TIO properly.)
[Proof that it is pristine](https://tio.run/##XZDRSsQwEEXf@xVDQZhAd9HCsiDrkx8gSB98ECVts9usMamZqe2y@O110kUUoZBm7p0zNxPneYLdCvK2D5bQhUOJteXxKUSkRntUxeVQqqhUnqVL8u8H37ANItyU26wPRKlKuu/dCcl8vDrj0TedjjhJ66/fqh/X9e32T/2o4JwBRLOAOJ7wTENNLP0FWPlUqkc9VuF@oUrAhK/Cox7xn1UGalqLGUt4gaOEF7T51A57Hckgm4nhDiRZ9pUk6zsTLRPK@AJymb4yMYaYJ11ljWakPlrPe8wfBoawh6sW0qttbZ1lK6mbzjRvpi2SIpjBsWBBtrWQ1s8@L0CWcuAOU6NEpOH98qvUXG42IJucvwE "R – Try It Online") (using [Nick Kennedy's checker](https://codegolf.stackexchange.com/a/182883/86301)).
Outputs 0 for falsey, and a positive value for truthy, which I understand [is acceptable](https://codegolf.meta.stackexchange.com/a/2194/86301) since R will interpret these as False and True.
Explanation: `bitwXor(a,b)` gives (as an integer) the bitwise XOR between `a` and `b`. To check whether it is a power of 2, check whether its log in base 2 is an integer. The function `dpois` gives the probability density function of the Poisson distribution: its value is 0 for non-integer values, and something positive for non-negative integers. The `T` is there because `dpois` requires a second argument (any positive real works, and `T` is interpreted as 1).
If we insist on outputting to distinct values, the following version outputs FALSE or TRUE in 42 bytes (thanks to Giuseppe for -8 bytes):
```
dpois(log2(bitwXor(scan(),scan())),T)%in%F
```
and [is also pristine](https://tio.run/##XVDfS8MwEH7vX3EUBhfohhbGQOaT4KsgffBBlKxL15s1qbmr6xj@7fWyIYoQSPh@3ZeL0zTCeg75tg/E2IVdiRuSw1OIyLX1aIrLZUxRmRn52X2eJSSZmsHXQkHZ63KV9YE5oWz7vjsiu4/Xznn0dWsjjur/1ZP5UV3drP7gewOnDCC6c5DEI5542LCovwDSYxIe7aEKd@dUbZniq/BoD/hPqgMtL1SMJbzAXn@g0e7TdtjbyA7FjQK3oM2yr0SRb10kYdTxBeQ6fe5iDDFPvMlqK8h9JC8N5g@DQGhgtoX0a9pQR0Laum5d/ea2RWI0ZuhEY0G3dU5aPPu8AF3KTlpMRq3Iw/vlacxULpegm5y@AQ). [Try it online!](https://tio.run/##VYtBCsIwFET3/xTZFPIxhiQY3Lj2BC6E0oVWIoGSSBsxpfTsMUlBcPWGmTdjMuS0J@bt@mC9o5xzXCCWTsBcMNgp1Bqm/ub@ZFyymItIdkTC3Lax69b0eHk70cE/Fb3b8Ln6kZYnRbYBkV2wsa45pxUMFYxIzJSMiEqtmDrUVCbxS5uVNalKUlqXR8oEqY7wBQ "R – Try It Online")
[Answer]
# [R](https://www.r-project.org/), 83 bytes
```
t(identical(sum(.<-as.double(intToBits(Reduce(bitwXor,scan())))),sum(T^el(.[-T]))))
```
[Try it online!](https://tio.run/##K/r/v0QjMyU1ryQzOTFHo7g0V0PPRjexWC8lvzQpJ1UjM68kJN8ps6RYIyg1pTQ5VSMps6Q8Ir9Ipzg5MU9DEwR0QJpC4lJzNPSidUNiQUL/DY3MFYxMTf8DAA "R – Try It Online")
[Proof that this is pristine](https://tio.run/##XZBhS8MwEIa/91ccBeECbdHBGMj8oj9AGP0gqJM0vdnMmtTk6jaGv71eCqIY8iHc@96TuzdM0xHWJeSMtiXH1uge4/iO1brUsWr92PSE1nHtby1H3FA7GsLG8uHBhyIa7VClU6Smeks9Vo9l/ZxKeZbkRN@NzrD1YjW4WC6Lq8VKZYOPMYlRD0N/wkgfLz05dKbTAY9C/G2z6sd1eb36U98rOGcAgWYQhxOe49hElv4CrFyV6kEfan83U33AhK/9Rh/wn1U@lI3FjAvYwl4WEDR9Sh6DDpGQ6chwAzJZ9pUk6zoKKRP5vpAAw6mkEHzIk64yoxnjECS6Heb3I4PfwUULaWvb2N6ylalNR@aN2iIpghl7FixIaDOpenJ5ARLKK3eYGmXEFPP8VGqSKEGinL4B "R – Try It Online")
Working around the fact that `as.integer`, `as.double` etc. are only a bit away from `is.integer`, `is.double` etc. was the hardest bit. In the end, using `sum(T^el(.[-T])` as a way of both generating a one and checking that `as.double` has returned a >1 length vector was the best I could do. The wrapping `t` is to handle the fact that otherwise `identical` can become `ide~tical`. 
[Answer]
# [Julia 0.7](http://julialang.org/), 20 bytes
```
(a,b)->ispow2(a⊻b)
```
[Try it online!](https://tio.run/##yyrNyUw0/59m@18jUSdJU9cus7ggv9xII/FR1@4kzf8FRZl5JTl5GmkaBjqGCiCgqamsEFJUWpJRyYWQNNQxwCNpaqRjZIJLEmiuAVSnW2JOcSq6pCFOYw11DI1w6TQyNdVBNfY/AA "Julia 0.7 – Try It Online")
Here is a [pristine validator](https://tio.run/##bdY9ThtBAIbhFlapOACyR0HMmFmwbIk/YRB0qUCiSJNix8ssHnu9rGfX2BYSSKkiInEAuAGUqRIK7mIkCi7hGOq3/J7vAm93mDqzNZslslCt0k@CSsVemlTmxhdWCilqRU0oWdd1JZSav6Uf2iA2ZdyZj8SkhQ1sdhaMW96MhBBBx@i2CvddkV@MGtJMb5/bKlgg3CasEGrCGqEilFXCQ8IBoSO0hDFhRGhSwi@Ee4RfCSXhOmFIqAXhEaEn7BImhBFhTNh2hIuEO4RVwpBwjVASqj5hQNgiXCFUhBuEmjC8JlwmXCfcJNwl3CM8INxXhN8IJ4SGsE/YI@wQuibhKWFMeEU4IhwQesKiTnhCGBGOCUtCTzggzDcIjwlvCM8Je4R9wozwYovwO@E54Q1hQTgkvCQcecIlQkG4S7hJWCdsEjY6hAuE24QVQk1YI1SEskp4SDggdISWMCaMCM074RvhK@H0kfCe8CfhHeEvwt@Etw@ET4R/CF8I/xL@I3wWhEeEnrBLmBBGhDFh2xEuEu4QVglDwjVCGXwUcO5dVkqTTWSiizx1pRzr1R/Zqu5Zm9t@Xk5an92slJrN/gM) that tries running each modified anonymous function against some input, and neither passes successfully. Note that the code has a multi-byte unicode character, and some possible outputs from bit flipping are not even included, as those produce invalid UTF-8 strings.
[Answer]
# [C# (Visual C# Interactive Compiler)](http://www.mono-project.com/docs/about-mono/releases/5.0.0/#csc), 37 bytes
```
a=>b=>a!=b&((a^b)&-(a^b)).Equals(a^b)
```
The `a=>b=>` part cannot be changed, or else the function is invalid.
In `a!=b`, the `=` cannot be changed since `int` cannot be converted to `bool`.
[Try it online!](https://tio.run/##bc89C4MwEAbgvb/CLpKDWHKHmdpkq1PHQrdCIkoDotSPob8@jQqFaLMk93DvHSmHrBycL6a2vLh25L@H7bpG61p5o7RV2hyVTRkzTwtptlxwur4n0wxL4c@HR@/G6ubaitVMAEOA2BACb00SMMp3POfFX92PDXORtkpSruu84JjMR@nk3k/j63NALrYiiVMeSYiJtacIf6xWwTiFHCnqCVt5lPoC "C# (Visual C# Interactive Compiler) – Try It Online")
[Answer]
# [C# (Visual C# Interactive Compiler)](http://www.mono-project.com/docs/about-mono/releases/5.0.0/#csc), ~~128~~ ~~101~~ ~~77~~ ~~70~~ ~~61~~ 74 bytes
*-27 bytes thanks to [Ascii-Only](https://codegolf.stackexchange.com/users/39244/ascii-only)*
```
a=>b=>{var d=Math.Log(a^b,(int)Math.E);return d.Equals((int)Math.Abs(d));}
```
[Try it online!](https://tio.run/##bc49C8IwEAbg3V/R8Q6qtMFONQGHOtXZTchHq4GaYpIKIv72GOsgto733Mt7J91SOh12g5EbdTf8omU6Dtr4VPR9x1jS0sApE5Q9btwmiu65P6/q/gT8KFKIQRylwtI2frAmUavqOvDOwXe5FQ4UYvkM5eJgtW9qbRpoIUfIIv9YhpEnlhcEgaxn/M5mf3VeEW/lZKqkKD4vhBc "C# (Visual C# Interactive Compiler) – Try It Online")
You have to be quite creative to get numbers in C# without using literals. Only uses ^ operator. Variables a,b are all more than 1 bit away from each other and everything else is a keyword/name. 
[Answer]
# JavaScript (ES6 in strict mode), 61 bytes
```
(y,z,e)=>eval(`(y${(e='^=z)*!(y&~-y)')!='^=z)*!(y&~-y)'||e}`)
```
[Try it online!](https://tio.run/##fY7RCoIwGEbvfYpfiLaFppN2FXbZE3Qdiv2WMVy4KcysVzcvgkJqtx@H851r3uW6aKqbCWt1wpG0GkGbpioM2XoSDZTpSG3QB8jSHXa5pBm1izvFlBzTnq18apfP0DLC/PkyDPjI2FioWiuJa6nOtKQAcQDAGYMogkPTmov15gSfiNhBcJEEkGxcDj698Ldjn0uNvwlnB586eOJwJEJ8Sv@8TKUgvonxBQ "JavaScript (Node.js) – Try It Online") or [Make sure that all modified programs are wrong](https://tio.run/##dZHdbuIwEIXveYohqtZ2Sa2CdqWqga4Quxd7V4lLREXqDpCK2NQxYQNNX52O89OllVaRovGcyTfHJ89xHmfKJlt3pc0TnvIURmDxZZdY5CxPmYhOigTqBrwIDyGK0R3m8YYveHFx5DhiD6ODuOzy4tvbVSGY6H7tvL5iuRBB1OksjeVbZ4l1HYEvhuDZcoN65dZVq9cTcOwA@NHHxNWjvhjCTVW0AwAa95Pa2UxK6UHz6FyYEW5O6tTZRK/k0pp0so6tl/j5iFRNd@y4gAfow3DoN4mallWGmw/ks0k0Z6zRtnGWkTioT84WjTXyv9OKlCqpgO0y9JxEORYF0PN1Q/hg9Ntznkq703/0xGiHfx1f/B91caSqDERUreP9H4MQBt/FIvQQZTF22FKOpWg2ltVbxU6tOVKWAM1iZXRmNvQ3zIoTNwReW6P7wU9gs21sMwoS0Fpj5wxuqUdOXZJi2xN0N0ZPD1A6UwfPxderXp8bSZbVGvGR3LmN4H48nf7@FYSfEnNra/bQav9gZac8nd4B)
[Answer]
# [Groovy](http://groovy-lang.org/), ~~47~~ 36 bytes
```
a,b->a.bitCount(a^b).equals(-~(a^a))
```
[Try it online!](https://tio.run/##Sy/Kzy@r/J9mW/0/USdJ1y5RLymzxDm/NK9EIzEuSVMvtbA0MadYQ7cOyE3U1Pxfy8VVUJSZV5KTp5CmYaBjqInENdQxQOGaGukYmaCIADUYoAugGmGoY2iELGBkago09T8A "Groovy – Try It Online")
Adapted version of [Kevin Cruijssen](https://codegolf.stackexchange.com/users/52210/kevin-cruijssen)'s Java answer.
[Answer]
# MATLAB, 37 bytes
```
@(c,e)eq(nnz(de2bi(bitxor(c,e))),eye)
```
Sorry, no TIO link, because I can't get the test suite to work under Octave. Thanks @ExpiredData for some helpful comments. 
Test suite:
```
program = '@(c,e)eq(nnz(de2bi(bitxor(c,e))),eye)';
number_of_characters = nnz(program);
success = [];
for character_counter = 0 : number_of_characters
    for bit_no = 1:8
        prog_temp = program;
        if(character_counter > 0)
            prog_temp(character_counter) = bitxor(double(prog_temp(character_counter)),2^(bit_no-1));
        elseif(bit_no<8) % Test the unmodified program once
            continue
        end
        try
            eval(prog_temp);
            eval('ans(2,3)');
            disp(prog_temp)
            success(end+1)=1;   
        catch
            success(end+1)=0;
        end 
    end
end
assert(nnz(success)==1)
```
[Answer]
# [Perl 6](https://github.com/nxadm/rakudo-pkg), ~~77~~ 43 bytes
*Thanks to Jo King for -33 bytes.*
```
{elems(i)eq(sum [+^](@_).polymod(+@_ xx*))}
```
This is equivalent to
```
{1 eq(sum [+^](@_).polymod(2 xx*))}
```
`1` was rewritten as `elems([""])`. `2` was rewritten as `sum(elems([""]),elems([""]))`; `elems(["",""])` might seem to work but `elems([""-""])` is also valid and seems to hang the tester.
[Try it online!](https://tio.run/##VVDRSsMwFH3uvuJSwmztWtrhFOw6OkZBULcHRZC5jeo6rWvsTDJYKf0W2V/4AfsRv6TepPNBktzk3Jx7Tm42CcvO6y1P4HYyvo4e7bsosscTO3oY3vgtWgDhEMBJXSZZQrmRmsmnwbcUptZ8ZoQL09nkWUHzpWGFC9jtTk2zqk8cwVKqqts/319YL9VAR/zUPuwDwn25635rlTPou@DhwNjrQvcMPPdvempistcDdwD2AEjcAfIMZUvjcQH6EQYDKNHHaKBZoXClnh6jtaveEa7xSLiTsyVvbOeYkoqpVFOJC4XfJdZkCcWScO0jCumUpDOw5gEYHlh9ZJl@wyJCsqjz8sa4TAlWKAHtX8vSXPiHveF2oGvqkth0MM4hYQzdVyynl0hq7iyLxOowGt6PrhpBbZms4m0moKwklAFXha0e/wJELuIMNjHn6ccrCtW/ "Perl 6 – Try It Online")
[Answer]
# [JavaScript (Node.js)](https://nodejs.org), 20 bytes
```
b=>d=>[i=d^b][i--&i]
```
[Try it online!](https://tio.run/##y0osSyxOLsosKNHNy09J/Z@TWqKQZvs/ydYuxdYuOtM2JS4pNjpTV1ctM/Z/cn5ecX5Oql5OfrpGmoaCgoGmhoKhpqaCvr5CSFFpSUYlF7oKQ6AKAzwqDE2NNDWMTPCZYQi0xRBqhltiTnEqdhV43QGU1TA0wmOGkakpwqU4bAG6VMEUWcV/AA "JavaScript (Node.js) – Try It Online")
[validator by Arnauld, modified (not use strict)](https://tio.run/##XVBda8IwFH33V1zKWBOqQWWDsapD3B72NvCxVIw1asQmLo3dhvS3dzf9cGUUSnLPxz05R57zLDHybAdKb0WZpzAFIz4v0gji56lPwzJBAKfeZjrbTmeRnG5XmziSg8G9jL2w19tpQ87WIGUYgjtMwEnYSai9PVSjIKBw7QE46kbamuoOE3iqDi0BQImvRb0wYow5ozjsAhHaxYgurZFqz3ZGp4sDNw4iXQpLmuncEgorGMFk4jbR2i2rAjcCdtRSEd9vsDPPMgTH9c2anyYa5r@oBBGR8xNBh4Z/U4zae54yc1HvaqGVFd@WrLtC7@6K2sKjYeVHRo9jSsYPdN13usQIbkUrvBa0WVJU/4Tb5EAElgXQ7Eq0yvQJ69Z7l6kPpE6DD4AX8KMzNxk2BcIYbWIfnnGG4axMRTujEICPXwCCWV03S@j/1w27QeSuWkNv1XRjeB/z5fLt1etDtyR7MPoLWuzPrOgVZfkL)
]