Symbolic diagonal matrix

`diag_(x, n = 1L, declare_symbols = TRUE, ...)`

- x
Character vector with diagonal

- n
Number of times

`x`

should be repeated- declare_symbols
Passed on to

`as_sym()`

when constructing symbolic matrix- ...
Passed on to

`rep(x, n, ...)`

```
if (has_sympy()) {
diag_(c(1,3,5))
diag_(c("a", "b", "c"))
diag_("a", 2)
diag_(vector_sym(4))
}
#> c: ⎡v₁ 0 0 0 ⎤
#> ⎢ ⎥
#> ⎢0 v₂ 0 0 ⎥
#> ⎢ ⎥
#> ⎢0 0 v₃ 0 ⎥
#> ⎢ ⎥
#> ⎣0 0 0 v₄⎦
```