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Just the vowels please

Given a sequence of letters, output only the vowels.

## 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 (there may be zero letters)

## Output
- A sequence of zero or more letters
- The sequence contains all of the vowels from the input, in the same order, and nothing else
- Vowels that appear more than once in the input must appear the same number of times in the output
- If there are no vowels in the input, the output is empty (a sequence of length zero)
- The characters that count as vowels for this challenge are `AEIOUaeiou`
- Case must be maintained - a vowel that is lower case in the input must be lower case in the output, and a vowel that is upper case in the input must be upper case in the output

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

```text
"" : ""
"q" : ""
"S" : ""
"Rhythm" : ""
"a" : "a"
"art" : "a"
"atr" : "a"
"rat" : "a"
"rta" : "a"
"tar" : "a"
"tra" : "a"
"emu" : "eu"
"eum" : "eu"
"meu" : "eu"
"mue" : "ue"
"uem" : "ue"
"ume" : "ue"
"AaeEIiOouU" : "AaeEIiOouU"
"aaaaaaaa" : "aaaaaaaa"
"bbbbbbbb" : ""
"abababab" : "aaaa"
"babababa" : "aaaa"
"aLtErNaTe" : "aEae"
```

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