Count Number of Special Subsequences LeetCode solution in Rust

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impl Solution {
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  pub fn count_special_subsequences(nums: Vec<i32>) -> i32 {
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    let mut dp = vec![0, 0, 0];
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    let mod_ = 1_000_000_007;
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    for &a in nums.iter() {
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      let a = a as usize;
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      dp[a] = ((dp[a] + dp[a]) % mod_ + (if a > 0 { dp[a - 1] } else { 1 })) % mod_;
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    }
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    dp[2]
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  }
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}