以下はCircom 2.0回路の例で、1つの入力 `a` を受けて `2 * a` を信号 `b` に出力する：

```circom
pragma circom 2.0.0;

template TwoInput() {  

   // Declaration of signals.  
   signal input a;  
   signal output b;  

   // Constraints.  
   b <== a + a;  
}
```
Here's an example of a Circom 2.0 circuit that takes one input `a` and outputs `2 * a` to signal `b`:

```circom
pragma circom 2.0.0;

template TwoInput() {  

   // Declaration of signals.  
   signal input a;  
   signal output b;  

   // Constraints.  
   b <== a + a;  
}
```