segment

auto r1 = [0, 1, 2, 3, 4, 5];
auto s = segment!2(r1);
assert(equal(s, [tuple(0, 1), tuple(1, 2), tuple(2, 3), tuple(3, 4), tuple(4, 5)][]));
assert(s.length == 5);         // .length
// back/popBack:
assert(equal(retro(s), retro([tuple(0, 1), tuple(1, 2), tuple(2, 3), tuple(3, 4), tuple(4, 5)][])));
assert(s[3] == tuple(3, 4));    // opIndex
s[3] = tuple(0, 0);             // opIndexAssign not ref opIndex
assert(s[2] == tuple(2, 0));    // it affects its neighbors.
assert(s[4] == tuple(0, 5));
assert(r1 == [0, 1, 2, 0, 0, 5][]); // affects r1 back (no .dup internally)


auto st = ["a","b","c","d","e","f"];
auto s2 = segment!3(st);
assert(s2.front == tuple("a","b","c"));


auto r1 = [0,1,2,3,4,5]; // regenerates r1
auto s3 = segment!1(r1);
assert(equal(s3, [tuple(0), tuple(1), tuple(2), tuple(3), tuple(4), tuple(5)][]));
auto r2 = map!"a*a"(r1);
auto s4 = segment!2(r2); // On a forward range
assert(equal(s4, [tuple(0,1), tuple(1,4), tuple(4,9), tuple(9,16), tuple(16,25)][]));


int[] e;
auto s5 = segment!2(e);
assert(s5.empty);