    "Abstraction of [[additive|Summable]] numeric types which
     support a unary operation `-x` producing the additive
     inverse of `x`. Every `Invertible` type supports a binary
     subtraction operation `x-y`.

         Integer negativeOne = -1;
         Float delta = x-y;

     A concrete class that implements this interface should be a
     mathematical _group_. That is, it should have an additive
     identity, denoted `0`, and satisfy:

     - `0+x == x+0 == x`
     - `x + -x == 0`

     Subtraction must be defined so that it is consistent with
     the additive inverse:

     - `x - y == x + -y`"
    see (`class Integer`, `class Float`)
    by ("Gavin")
    shared interface Invertible<Other> of Other
            satisfies Summable<Other>
        given Other satisfies Invertible<Other> {

        "The additive inverse of this value."
        shared formal Other negated;

        "The difference between this number and the given
         number. Must produce the value `x + -y`."
        shared default Other minus(Other other) => this + -other;

    }