Retrieves the value at a specific index by isolating it from the prefix sum. This operation determines the value at `index` by subtracting the prefix sum up to `index - 1` from the prefix sum up to `index`. # Arguments `index` - The zero-based index of the element to retrieve. # Returns A `Result` containing the value at the specified index (`Ok(value)`) or an error (`FenwickTreeError::IndexO
(&self, index: usize)
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