Summator.opOpAssign

import std.math, std.algorithm, std.range;
auto r1 = iota(500).map!(a => 1.7L.pow(a+1) - 1.7L.pow(a));
auto r2 = iota(500, 1000).map!(a => 1.7L.pow(a+1) - 1.7L.pow(a));
Summator!real s1 = 0, s2 = 0.0;
foreach (e; r1) s1 += e;
foreach (e; r2) s2 -= e;
s1 -= s2;
s1 -= 1.7L.pow(1000);
assert(s1.sum() == -1);