    "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.

     In general, the addition operation should be a
     binary associative operation."
    see (`class String`, `interface Numeric`)
    by ("Gavin")
    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."
        shared formal Other plus(Other other);

    }