Function integer_part 

pub fn integer_part<V: TowerValue>(
    v: &V,
    mode: Rounding,
    recurse: &dyn Fn(V) -> Result<V, String>,
) -> Option<Result<V, String>>

The integer part of an exact value under mode; None when v is not one (the tower handles it). recurse is the builtin itself, for the standard values inside a dependence or a provisional answer.