groupBy

import std.stdio : writeln;
import std.algorithm : sort;
import std.typecons : tuple;
auto xs = [
    tuple("a", 1.0),
    tuple("b", 3.0),
    tuple("a", 2.0),
    tuple("b", 4.0)];
auto grouped = xs.groupBy!((a) => a[0]);
assertEqual( grouped.keys.length, 2 );
assertEqual( grouped.keys.sort(), ["a","b"].sort() );