This is what the trace output of Carbon Explorer was like after the Google Summer of Code 23 project was completed.

new_default.trace

* * * * * * * * * *  starting execution  * * * * * * * * * *
------------------------------------------------------------

- - - - -  initializing globals  - - - - -
------------------------------------------
>[] stack-push: DeclarationAction pos: 0 `namespace N` (./testfile.carbon:3)
->> step DeclarationAction pos: 0 `namespace N` (./testfile.carbon:3) --->

*** declaration at (./testfile.carbon:3)
```
namespace N;
```
<[] stack-pop:  DeclarationAction pos: 0 `namespace N` (./testfile.carbon:3)
>[] stack-push: DeclarationAction pos: 0 `fn N.Add` (./testfile.carbon:7)
->> step DeclarationAction pos: 0 `fn N.Add` (./testfile.carbon:7) --->

*** declaration at (./testfile.carbon:7)
```
fn Add (a: i32, b: i32) -> i32
{
  return (a + b);
}
```
<[] stack-pop:  DeclarationAction pos: 0 `fn N.Add` (./testfile.carbon:7)
>[] stack-push: DeclarationAction pos: 0 `fn Main` (./testfile.carbon:12)
->> step DeclarationAction pos: 0 `fn Main` (./testfile.carbon:12) --->

*** declaration at (./testfile.carbon:12)
```
fn Main () -> i32
{
  var sum: auto = N.Add(1, 2);
  return sum;
}
```
<[] stack-pop:  DeclarationAction pos: 0 `fn Main` (./testfile.carbon:12)

- - - - -  calling main function  - - - - -
-------------------------------------------
>[] stack-push: ValueExpressionAction pos: 0 `Main()` (<Main()>:0)
->> step ValueExpressionAction pos: 0 `Main()` (<Main()>:0) --->
>[] stack-push: ExpressionAction pos: 0 `Main()` (<Main()>:0)
->> step ExpressionAction pos: 0 `Main()` (<Main()>:0) --->
>[] stack-push: ValueExpressionAction pos: 0 `Main` (<Main()>:0)
->> step ValueExpressionAction pos: 0 `Main` (<Main()>:0) --->
>[] stack-push: ExpressionAction pos: 0 `Main` (<Main()>:0)
->> step ExpressionAction pos: 0 `Main` (<Main()>:0) --->
<[] stack-pop:  ExpressionAction pos: 0 `Main` (<Main()>:0)
->> step ValueExpressionAction pos: 1 `Main` results: [`fun<Main>`]  (<Main()>:0) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `Main` results: [`fun<Main>`]  (<Main()>:0)
->> step ExpressionAction pos: 1 `Main()` results: [`fun<Main>`]  scope: [] (<Main()>:0) --->
-() calling function: fun<Main>
=== match pattern `()`
    from value expression with value `()`
>[] stack-push: ScopeAction pos: 0  scope: [] (None)
>[] stack-push: StatementAction pos: 0 `{ ... }` (./testfile.carbon:12)
->> step StatementAction pos: 0 `{ ... }` (./testfile.carbon:12) --->

*** statement at (./testfile.carbon:12)
```
{
  var sum: auto = N.Add(1, 2);
  return sum;
}
```
>[] stack-push: StatementAction pos: 0 `var ... = ...;` (./testfile.carbon:10)
->> step StatementAction pos: 0 `var ... = ...;` (./testfile.carbon:10) --->

*** statement at (./testfile.carbon:10)
```
var sum: auto = N.Add(1, 2);
```
++# memory-alloc: #1 `Uninit<i32>` uninitialized
>[] stack-push: ExpressionAction pos: 0 `N.Add(1, 2)` (./testfile.carbon:10)
->> step ExpressionAction pos: 0 `N.Add(1, 2)` (./testfile.carbon:10) --->
>[] stack-push: ValueExpressionAction pos: 0 `N.Add` (./testfile.carbon:10)
->> step ValueExpressionAction pos: 0 `N.Add` (./testfile.carbon:10) --->
>[] stack-push: ExpressionAction pos: 0 `N.Add` (./testfile.carbon:10)
->> step ExpressionAction pos: 0 `N.Add` (./testfile.carbon:10) --->
<[] stack-pop:  ExpressionAction pos: 0 `N.Add` (./testfile.carbon:10)
>[] stack-push: ExpressionAction pos: 0 `Add` (./testfile.carbon:10)
->> step ExpressionAction pos: 0 `Add` (./testfile.carbon:10) --->
<[] stack-pop:  ExpressionAction pos: 0 `Add` (./testfile.carbon:10)
->> step ValueExpressionAction pos: 1 `N.Add` results: [`fun<N.Add>`]  (./testfile.carbon:10) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `N.Add` results: [`fun<N.Add>`]  (./testfile.carbon:10)
->> step ExpressionAction pos: 1 `N.Add(1, 2)` results: [`fun<N.Add>`]  scope: [] (./testfile.carbon:10) --->
>[] stack-push: ExpressionAction pos: 0 `1` (./testfile.carbon:10)
->> step ExpressionAction pos: 0 `1` (./testfile.carbon:10) --->
<[] stack-pop:  ExpressionAction pos: 0 `1` (./testfile.carbon:10)
->> step ExpressionAction pos: 2 `N.Add(1, 2)` results: [`fun<N.Add>`, `1`]  scope: [] (./testfile.carbon:10) --->
>[] stack-push: ExpressionAction pos: 0 `2` (./testfile.carbon:10)
->> step ExpressionAction pos: 0 `2` (./testfile.carbon:10) --->
<[] stack-pop:  ExpressionAction pos: 0 `2` (./testfile.carbon:10)
->> step ExpressionAction pos: 3 `N.Add(1, 2)` results: [`fun<N.Add>`, `1`, `2`]  scope: [] (./testfile.carbon:10) --->
-() calling function: fun<N.Add>
=== match pattern `(Placeholder<a>, Placeholder<b>)`
    from value expression with value `(1, 2)`
=== match pattern `Placeholder<a>`
    from value expression with value `1`
=== match pattern `Placeholder<b>`
    from value expression with value `2`
>[] stack-push: ScopeAction pos: 0  scope: [`a: i32`: `1`, `b: i32`: `2`] (None)
>[] stack-push: StatementAction pos: 0 `{ ... }` (./testfile.carbon:7)
->> step StatementAction pos: 0 `{ ... }` (./testfile.carbon:7) --->

*** statement at (./testfile.carbon:7)
```
{
  return (a + b);
}
```
>[] stack-push: StatementAction pos: 0 `return ...;` (./testfile.carbon:6)
->> step StatementAction pos: 0 `return ...;` (./testfile.carbon:6) --->

*** statement at (./testfile.carbon:6)
```
return (a + b);
```
>[] stack-push: ValueExpressionAction pos: 0 `(a + b)` (./testfile.carbon:6)
->> step ValueExpressionAction pos: 0 `(a + b)` (./testfile.carbon:6) --->
>[] stack-push: ExpressionAction pos: 0 `(a + b)` (./testfile.carbon:6)
->> step ExpressionAction pos: 0 `(a + b)` (./testfile.carbon:6) --->
>[] stack-push: ValueExpressionAction pos: 0 `a` (./testfile.carbon:6)
->> step ValueExpressionAction pos: 0 `a` (./testfile.carbon:6) --->
>[] stack-push: ExpressionAction pos: 0 `a` (./testfile.carbon:6)
->> step ExpressionAction pos: 0 `a` (./testfile.carbon:6) --->
<[] stack-pop:  ExpressionAction pos: 0 `a` (./testfile.carbon:6)
->> step ValueExpressionAction pos: 1 `a` results: [`1`]  (./testfile.carbon:6) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `a` results: [`1`]  (./testfile.carbon:6)
->> step ExpressionAction pos: 1 `(a + b)` results: [`1`]  (./testfile.carbon:6) --->
>[] stack-push: ValueExpressionAction pos: 0 `b` (./testfile.carbon:6)
->> step ValueExpressionAction pos: 0 `b` (./testfile.carbon:6) --->
>[] stack-push: ExpressionAction pos: 0 `b` (./testfile.carbon:6)
->> step ExpressionAction pos: 0 `b` (./testfile.carbon:6) --->
<[] stack-pop:  ExpressionAction pos: 0 `b` (./testfile.carbon:6)
->> step ValueExpressionAction pos: 1 `b` results: [`2`]  (./testfile.carbon:6) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `b` results: [`2`]  (./testfile.carbon:6)
->> step ExpressionAction pos: 2 `(a + b)` results: [`1`, `2`]  (./testfile.carbon:6) --->
<[] stack-pop:  ExpressionAction pos: 2 `(a + b)` results: [`1`, `2`]  (./testfile.carbon:6)
->> step ValueExpressionAction pos: 1 `(a + b)` results: [`3`]  (./testfile.carbon:6) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `(a + b)` results: [`3`]  (./testfile.carbon:6)
->> step StatementAction pos: 1 `return ...;` results: [`3`]  (./testfile.carbon:6) --->

*** statement at (./testfile.carbon:6)
```
return (a + b);
```
--> memory-write: #1 `3`
<[] stack-pop:  StatementAction pos: 1 `return ...;` results: [`3`]  (./testfile.carbon:6)
<[] stack-pop:  StatementAction pos: 1 `{ ... }` scope: [] (./testfile.carbon:7)
<[] stack-pop:  ScopeAction pos: 0  scope: [`a: i32`: `1`, `b: i32`: `2`] (None)
>[] stack-push: CleanUpAction pos: 0  scope: [`a: i32`: `1`, `b: i32`: `2`] (stack cleanup:1)
>[] stack-push: CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step CleanUpAction pos: 0  scope: [] (stack cleanup:1) --->
<[] stack-pop:  CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step CleanUpAction pos: 0  scope: [`a: i32`: `1`, `b: i32`: `2`] (stack cleanup:1) --->
<[] stack-pop:  CleanUpAction pos: 0  scope: [`a: i32`: `1`, `b: i32`: `2`] (stack cleanup:1)
->> step ExpressionAction pos: 4 `N.Add(1, 2)` results: [`fun<N.Add>`, `1`, `2`, `3`]  scope: [] (./testfile.carbon:10) --->
<[] stack-pop:  ExpressionAction pos: 4 `N.Add(1, 2)` results: [`fun<N.Add>`, `1`, `2`, `3`]  scope: [] (./testfile.carbon:10)
>[] stack-push: CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step CleanUpAction pos: 0  scope: [] (stack cleanup:1) --->
<[] stack-pop:  CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step StatementAction pos: 1 `var ... = ...;` results: [`3`]  (./testfile.carbon:10) --->

*** statement at (./testfile.carbon:10)
```
var sum: auto = N.Add(1, 2);
```
<-- memory-read: #1 `3`
=== match pattern `Placeholder<sum>`
    from initializing expression with value `3`
<[] stack-pop:  StatementAction pos: 1 `var ... = ...;` results: [`3`]  (./testfile.carbon:10)
->> step StatementAction pos: 1 `{ ... }` scope: [`sum: auto`: `lval<Allocation(1)>`] (./testfile.carbon:12) --->

*** statement at (./testfile.carbon:12)
```
{
  var sum: auto = N.Add(1, 2);
  return sum;
}
```
>[] stack-push: StatementAction pos: 0 `return ...;` (./testfile.carbon:11)
->> step StatementAction pos: 0 `return ...;` (./testfile.carbon:11) --->

*** statement at (./testfile.carbon:11)
```
return sum;
```
>[] stack-push: ValueExpressionAction pos: 0 `sum` (./testfile.carbon:11)
->> step ValueExpressionAction pos: 0 `sum` (./testfile.carbon:11) --->
>[] stack-push: ExpressionAction pos: 0 `sum` (./testfile.carbon:11)
->> step ExpressionAction pos: 0 `sum` (./testfile.carbon:11) --->
<-- memory-read: #1 `3`
<[] stack-pop:  ExpressionAction pos: 0 `sum` (./testfile.carbon:11)
->> step ValueExpressionAction pos: 1 `sum` results: [`3`]  (./testfile.carbon:11) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `sum` results: [`3`]  (./testfile.carbon:11)
->> step StatementAction pos: 1 `return ...;` results: [`3`]  (./testfile.carbon:11) --->

*** statement at (./testfile.carbon:11)
```
return sum;
```
<[] stack-pop:  StatementAction pos: 1 `return ...;` results: [`3`]  (./testfile.carbon:11)
<[] stack-pop:  StatementAction pos: 2 `{ ... }` scope: [`sum: auto`: `lval<Allocation(1)>`] (./testfile.carbon:12)
<[] stack-pop:  ScopeAction pos: 0  scope: [] (None)
>[] stack-push: CleanUpAction pos: 0  scope: [] (stack cleanup:1)
>[] stack-push: CleanUpAction pos: 0  scope: [`sum: auto`: `lval<Allocation(1)>`] (stack cleanup:1)
->> step CleanUpAction pos: 0  scope: [`sum: auto`: `lval<Allocation(1)>`] (stack cleanup:1) --->
<-- memory-read: #1 `3`
>[] stack-push: DestroyAction pos: 0  (None)
->> step DestroyAction pos: 0  (None) --->
<[] stack-pop:  DestroyAction pos: 0  (None)
->> step CleanUpAction pos: 1  scope: [`sum: auto`: `lval<Allocation(1)>`] (stack cleanup:1) --->
--# memory-dealloc: #1 `3`
->> step CleanUpAction pos: 2  scope: [`sum: auto`: `lval<Allocation(1)>`] (stack cleanup:1) --->
<[] stack-pop:  CleanUpAction pos: 2  scope: [`sum: auto`: `lval<Allocation(1)>`] (stack cleanup:1)
->> step CleanUpAction pos: 0  scope: [] (stack cleanup:1) --->
<[] stack-pop:  CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step ExpressionAction pos: 2 `Main()` results: [`fun<Main>`, `3`]  scope: [] (<Main()>:0) --->
<[] stack-pop:  ExpressionAction pos: 2 `Main()` results: [`fun<Main>`, `3`]  scope: [] (<Main()>:0)
>[] stack-push: CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step CleanUpAction pos: 0  scope: [] (stack cleanup:1) --->
<[] stack-pop:  CleanUpAction pos: 0  scope: [] (stack cleanup:1)
->> step ValueExpressionAction pos: 1 `Main()` results: [`3`]  (<Main()>:0) --->
<[] stack-pop:  ValueExpressionAction pos: 1 `Main()` results: [`3`]  (<Main()>:0)
==> interpreter result: 3