How to add Parsing and Evaluation of the min and max functions

[mpp]

Hi!

I forked the repo and I'm trying to add min and max functions.
I'm completely new to F# and I'm struggling with a compile error (that was obviously not present before my code).

As a total noob I approached the problem by trying to do the same as it was done for the function atan2 as both min and max are functions that take 2 arguments (at least?).

Is this the correct way to do it?
Can you give me any suggestion on how to fix the issue?

This is the link to the commit I made:
eb74512

I get the error

[...]\Workspace\mathnet-symbolics\src\Symbolics\Polynomial.fs(123,69): error FS0001: Type mismatch. Expecting a�    'System.Numerics.BigInteger * (Expression * System.Numerics.BigInteger) list -> System.Numerics.BigInteger * (Expression * System.Numerics.BigInteger) list -> System.Numerics.BigInteger * (Expression * System.Numerics.BigInteger) list'    �but given a�    'BigInteger * (Expression * Expression) list -> BigInteger * (Expression * Expression) list -> System.Numerics.BigInteger * (Expression * Expression) list'    �The type 'System.Numerics.BigInteger' does not match the type 'Expression'

    [<CompiledName("CommonMonomialFactors")>]
    let commonMonomialFactors xs =
        let normalizePowers = List.map (fun (r, p) -> pow r (fromInteger p))
        let denormalizePowers = List.map (function
            | PosIntPower (r, (Integer n)) -> (r, n.Numerator)
            | x -> (x, 1I))
        let monomialFactors x =
            let n, xs = Algebraic.factorsInteger x
            n, denormalizePowers xs
        let intersect ((n1:BigInteger),x1) ((n2:BigInteger),x2) =
            let n' = Euclid.GreatestCommonDivisor (n1, n2)
            let x' = x1 |> List.choose (fun (r1, p1) -> x2 |> List.tryPick (fun (r2,p2) -> if r2 = r1 then Some (r1, min p1 p2) else None))
            n', x'
        let (n, x') = xs |> List.map monomialFactors |> List.reduce intersect ///// <- Compile error on _intersect_
        (fromInteger n)::(normalizePowers x') |> product

[mpp]

I found the issue and "solved" it. I had to rename the operators min and max to min2 and max2:

    let min2 (x:Expression) (y:Expression) : Expression = FunctionN (Min, [x;y])
    let max2 (x:Expression) (y:Expression) : Expression = FunctionN (Max, [x;y])
    let arctan2 (x:Expression) (y:Expression) : Expression = FunctionN (Atan2, [x;y])

As they were conflicting with the operators in the Core module.

Is this approach correct and acceptable for a pull request?
Are the tests that I added exhaustive?

The code is @ mpp@d602aae