pub trait Semigroup {
fn mappend(self, other: Self) -> Self;
}Expand description
A Semigroup is a type with an associative operation. In plain terms, this
means you can take two values of this type and add them together into a
different value of the same type. The most obvious example of this is
addition of numbers: 2 + 2 = 4, another is string concatenation:
"Hello " + "Joe" = "Hello Joe".
Semigroups must follow the law of associativity:
(x + y) + z = x + (y + z)
A Semigroup differs from std::ops::Add in that Add can be defined
for any collection of types, eg. you could define Add for a type A which
takes a second argument of type B and returns a third type C, whereas a
Semigroup only deals with a single type A.