Function unary 

pub fn unary<'a, D: DataT>(
    cv: Cv<'a, D>,
    f: impl Fn(D::V<'a>, D::V<'a>) -> ValR<D::V<'a>> + 'a,
) -> ValXs<'a, D::V<'a>>

Create a filter that takes a single variable argument and whose output is given by the function f that takes the input value and the value of the variable.