MathF not found in UnityEngine?

Hi,
I have this chunk of code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class MouseLook : MonoBehaviour {
	public enum RotationAxes{
		MouseXAndY = 0,
		MouseX = 1,
		MouseY = 2
	}
	public RotationAxes axes = RotationAxes.MouseXAndY;

	public float sensitivityHor = 9f;
	public float sensitivityVert = 9f;

	public float minimumVert = -45f;
	public float maximumVert = 45f;

	private float _rotationX = 0;

	// Use this for initialization
	void Start () {

	}

	// Update is called once per frame
	void Update () {
		if (axes == RotationAxes.MouseX) {
			transform.Rotate(0, Input.GetAxis("Mouse X") * sensitivityHor, 0);
		} else if (axes == RotationAxes.MouseY) {
			_rotationX -= Input.GetAxis("Mouse Y") * sensitivityVert;
			_rotationX = MathF.Clamp (_rotationX, minimumVert, maximumVert);

			float rotationY = transform.localEulerAngles.y;

			transform.localEulerAngles = new Vector3 (_rotationX, rotationY, 0);
		} else {
			//both horizontal and vertical rotation
		}
	}
}

Nevertheless i get this compilation error in console:

Assets/Scripts/MouseLook.cs(32,17): error CS0103: The name `MathF' does not exist in the current context

Moreover, if I add Unity.Engine. to Mathf.Clamp I get:

Assets/Scripts/MouseLook.cs(32,29): error CS0234: The type or namespace name `MathF' does not exist in the namespace `UnityEngine'. Are you missing an assembly reference?

Nevertheless, according to MathF Unity 5.5 (my version) online reference MathF is contained in Unity Engine: Unity - Scripting API: Mathf

Is there something wrong with my UnityEngine library? Someone else had this issue? In past versions I had no problem about this.

It’s Mathf not MathF

MathF is totally a thing in C#. Mathf is the Unity math library.
I was wondering why MathF is somehow unavailable at Unity, even last updates with .NET 4.0…