open MathNet.Symbolics

open MathNet.Symbolics.Operators

​

let expr = Infix.parseOrUndefined("sin(x) * y + cos(x) + cos( vi / 2.0 ) *  cos((v + mu) * step / 2.0)")

​

​

let getsymbols(expr:Expression) = 

    let rec loop (expr:Expression) (acc:Symbol list) = 

        match expr with

        |Expression.Identifier(x) -> x::acc

        |Expression.Function(x,y) -> loop y acc

        |Expression.Product(x)

        |Expression.Sum(x) -> x |> List.fold(fun t el -> t @ loop el acc) acc

        |Expression.Power(x,y) -> loop x acc @ loop y acc

        |Expression.Number(_) -> acc

        | _ -> acc

    loop expr []

    |> Set.ofList

    |> Set.toList

​

getsymbols(expr) |> printfn "%A"