1 | initial version |

Symbolic expressions is a big bundle of stuff. For example, you can notice that `pi`

is also symbolic expression:

```
sage: type(pi)
<type 'sage.symbolic.expression.Expression'>
```

Some equations are also symbolic expressions:

```
sage: type(x^2 + 1 == x)
<type 'sage.symbolic.expression.Expression'>
```

You can notice, that `g`

and `h`

have the same type, but not the same parent:

```
sage: g.parent()
Callable function ring with arguments (x, y)
sage: h.parent()
Symbolic Ring
```

Since `g`

is callable, the way to evaluate it is supposed to be:

```
sage: g(0,1)
1
```

If you want do deal uniformly with `g`

and `h`

, you can use the `.subs()`

method:

```
sage: h.subs({x:0,y:1})
1
sage: g.subs({x:0,y:1})
(x, y) |--> 1
```

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