The docs say:

```
help?> isposdef
search: isposdef isposdef!
isposdef(A) -> Bool
Test whether a matrix is positive definite.
Example
≡≡≡≡≡≡≡≡≡
julia> A = [1 2; 2 50]
2×2 Array{Int64,2}:
1 2
2 50
julia> isposdef(A)
true
```

But the following matrix (c.f. Wikipedia) is not correctly identified as positive definite:

```
julia> M = [1 1;-1 1]
2×2 Array{Int64,2}:
1 1
-1 1
julia> isposdef(M)
false
```

There is some confusion in the literature about whether a positive definite matrix is also symmetric (or more generally Hermitian), but this could be clarified by introducing `isspd`

(or some other name).