"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); }