    """Abstraction of types which support a binary addition
       operator `x + y`. For [[numeric types|Numeric]], this is
       just familiar numeric addition. For [[String]], it is
       string concatenation.

           Integer next = current + 1;
           String helloWorld = "hello" + " " + "world";

       A concrete class that implements this interface should be
       a mathematical _semigroup_. That is, the addition
       operation should be associative, satisfying:

       - `(x+y)+z == x+(y+z)`

       A `Summable` type might be a _monoid_, that is, a
       semigroup with an additive identity element, usually
       denoted `0`, but this is not required. For example:

       - `String` is a monoid with identity element `""`,
       - `Float` is a monoid with identity element `0.0`, and
       - `Integer` is a monoid with identity element `0`.

       For any monoid, the addition operation must satisfy:

       - `x + 0 == x`"""
    see (`class String`, `interface Numeric`)
    by ("Gavin")
    tagged("Numbers", "Strings")
    shared interface Summable<Other> of Other
            given Other satisfies Summable<Other> {

        "The result of adding the given value to this value.
         This operation should never perform any kind of
         mutation upon either the receiving value or the
         argument value.

         For any two instances `x` and `y` of a type that
         implements `Summable`, `x.plus(y)` may be written as:

             x + y"
        shared formal Other plus(Other other);

    }