First steps using `Symbolics.jl`

.

I’d like to program an elementary calculus trainer:

- An expression is shown
- As solution, the expanded expression is shown
- As proof, the expression is evaluated with numerical values.

What is the best way for the last point, using `substitute`

or `build_function`

?

```
using Symbolics
@variables a b
f = (a + 2*b)^2
f_exp = expand(f)
println(f) # (a + 2b)^2
println(f_exp) # a^2 + 4(b^2) + 4a*b
# f_num = build_function(f, a, b) # not working
# f_num(2, 3) # 64
```