Is isogram?

Determine if a word is an isogram — no letter appears more than once.

strings case-insensitive set validation

An isogram is a word or phrase in which no letter occurs more than once. Your task is to implement the function `is_isogram(word)` that returns `True` if the given word is an isogram, and `False` otherwise.

Rules:
- The check is case-insensitive: 'A' and 'a' are considered the same letter.
- Ignore all non-alphabetic characters (spaces, hyphens, digits, punctuation, etc.) — they do not count.
- An empty word or a word with no alphabetic characters is considered an isogram.

For example: `'isogram'` is an isogram, `'documentarily'` is an isogram, but `'alpha'` is not because 'a' appears twice. `'Alphabet'` is not an isogram (case-insensitive). `'six-year-old'` is an isogram because only letters matter.

Implement the function exactly as specified. Do not include extra output or input handling.

Constraints

Input is a string. Length of input is not explicitly limited but will be manageable in typical testing. Output must be a boolean.

Example

>>> is_isogram('isogram')
True
>>> is_isogram('alpha')
False
>>> is_isogram('Alphabet')
False
>>> is_isogram('six-year-old')
True
>>> is_isogram('')
True

8 points ~12 min

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Hints

Convert the word to lowercase (or uppercase) before comparing letters.

Use a set to track letters you have already seen.

Filter out non-alphabetic characters using `str.isalpha()`.

Test Results

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