Given a string consisting of English alphabetic characters (both uppercase and lowercase), count the number of distinct vowels present. Vowels are 'a', 'e', 'i', 'o', 'u', and the count is case-insensitive (i.e., 'A' and 'a' are considered the same vowel).
Input: "Hello"
Output: 2
Explanation: The string "Hello" contains vowels 'e' and 'o', which are distinct, so the count is 2.
Input: "AEiou"
Output: 5
Explanation: All five vowels ('a', 'e', 'i', 'o', 'u') are present, and since the count is case-insensitive, they are all distinct, resulting in a count of 5.
Input: "bcdfg"
Output: 0
Explanation: The string contains no vowels, so the count of distinct vowels is 0.
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