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;  
}
```
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