Computes the sum of elements from the start of the tree up to a specified index. This operation efficiently calculates the prefix sum using the tree structure. # Arguments `index` - The zero-based index up to which the sum should be computed. # Returns A `Result` containing the prefix sum (`Ok(sum)`) or an error (`FenwickTreeError::IndexOutOfBounds`) if the index is out of bounds.
(&self, index: usize)
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