+is_deeply(cumulate({ 1 => 1 }, 1, 0), { 1 => 1 }, 'cumulate ref, non-zero, zero');
+is_deeply(cumulate({ 1 => 1 }, 2, 1), { 1 => 2 }, 'cumulate ref, 2, 1');
+is_deeply(cumulate({ 1 => 0.5, 2 => 0.5 }, 1, 0.5), { 1 => 0.5, 2 => 0.5 },
+ 'cumulate ref, 1, frac');
+is_deeply(cumulate({ 1 => 0.5, 2 => 0.5 }, 2, 0.5), { 1 => 0.75, 2 => 0.75 },
+ 'cumulate ref, 1, frac');