Update the docs of cancel_{minus,plus}

src/basic.rs

@@ -4,26 +4,71 @@ use std::{cmp::Ordering, unreachable};
 // NOTE: `neg`, `add`, `sub`, `mul` and `div` are implemented in arith.rs
 
 impl Interval {
-    /// When `self` and `rhs` are bounded, returns the tightest
-    /// interval `z` such that `rhs` + `z` ⊇ `self` if such `z`
-    /// exists, which is the case iff the width of `self` is greater
-    /// than or equal to the width of `rhs`.  If `self` or `rhs` is
-    /// unbounded, or such `z` does not exist, returns
-    /// [`ENTIRE`][Self::ENTIRE].
+    /// Returns the tightest interval `z` such that `rhs` $+$ `z` $⊇$ `self`,
+    /// if both `self` and `rhs` are bounded and the width of `self` is greater than or equal to
+    /// that of `rhs`. Otherwise, returns [`Interval::ENTIRE`].
     ///
-    /// Mathematically, $\operatorname{cancel_minus}(y+z, y) = z$
-    /// should hold.  However, with floating-point arithmetic, this
-    /// function may return a slight overestimation of $z$.  Moreover,
-    /// when `x.cancel_minus(y)` is not [`ENTIRE`][Self::ENTIRE], one has
-    /// `x.cancel_minus(y)` ⊆ `x` - `y` but generally not the equality.
+    /// Even when `x.cancel_minus(y)` is not [`Interval::ENTIRE`], its value is generally
+    /// different from `x - y`, as the latter gives the tightest enclosure of `x` $-$ `y`,
+    /// while the former does not always enclose `x` $-$ `y`.
+    /// In such case, `x.cancel_minus(y)` $⊆$ `x - y` holds, but generally not the equality.
     ///
-    /// # Example
+    /// # Examples
+    ///
+    /// Getting an enclosure of a partial sum omitting a single term from their total:
+    ///
+    /// ```
+    /// use inari::*;
+    /// let x = interval!("[0.1, 0.2]").unwrap();
+    /// let y = interval!("[0.3, 0.4]").unwrap();
+    /// let z = interval!("[0.5, 0.6]").unwrap();
+    /// let sum = x + y + z;
+    /// assert!((y + z).subset(sum.cancel_minus(x)));
+    /// assert!((x + z).subset(sum.cancel_minus(y)));
+    /// assert!((x + y).subset(sum.cancel_minus(z)));
+    /// ```
+    ///
+    /// `sum.cancel_minus(x)` is a subset of `sum - x`:
+    ///
+    /// ```
+    /// # use inari::*;
+    /// # let x = interval!("[0.1, 0.2]").unwrap();
+    /// # let y = interval!("[0.3, 0.4]").unwrap();
+    /// # let z = interval!("[0.5, 0.6]").unwrap();
+    /// # let sum = x + y + z;
+    /// assert!(sum.cancel_minus(x).subset(sum - x));
+    /// ```
+    ///
+    /// But the inverse does not hold in general:
+    ///
+    /// ```should_panic
+    /// # use inari::*;
+    /// # let x = interval!("[0.1, 0.2]").unwrap();
+    /// # let y = interval!("[0.3, 0.4]").unwrap();
+    /// # let z = interval!("[0.5, 0.6]").unwrap();
+    /// # let sum = x + y + z;
+    /// assert!((sum - x).subset(sum.cancel_minus(x)));
+    /// ```
+    ///
+    /// `sum.cancel_minus(x)`, etc. returns [`Interval::ENTIRE`] when `sum` is unbounded:
+    ///
+    /// ```
+    /// use inari::*;
+    /// let x = const_interval!(1.0, 2.0);
+    /// let y = const_interval!(3.0, f64::MAX);
+    /// let sum = x + y;
+    /// assert_eq!(sum.cancel_minus(x), Interval::ENTIRE);
+    /// assert_eq!(sum.cancel_minus(y), Interval::ENTIRE);
+    /// ```
+    ///
+    /// `sum.cancel_minus(x)` returns [`Interval::ENTIRE`] when the width of “`sum`” is
+    /// less than that of `x`:
     ///
     /// ```
     /// use inari::*;
-    /// let y = const_interval!(0.0, 1.0);
-    /// let z = const_interval!(3.0, 4.0);
-    /// assert_eq!((y + z).cancel_minus(y), z);
+    /// let x = const_interval!(1.0, 2.0);
+    /// let sum = const_interval!(4.0, 4.5);
+    /// assert_eq!(sum.cancel_minus(x), Interval::ENTIRE);
     /// ```
     #[must_use]
     pub fn cancel_minus(self, rhs: Self) -> Self {
@@ -128,9 +173,9 @@ impl Interval {
         }
     }
 
-    /// `x.cancel_plus(y)` is `x.cancel_minus(-y)`.
+    /// `x.cancel_plus(y)` is equivalent to `x.cancel_minus(-y)`.
     ///
-    /// See [`cancel_minus`][Self::cancel_minus] for more information.
+    /// See [`Interval::cancel_minus`] for more information.
     #[inline]
     #[must_use]
     pub fn cancel_plus(self, rhs: Self) -> Self {