fillFlat

import std.array;

auto app = appender!(int[])();
fillFlat!int( app, [1,2], 3,
                   [[4,5],[6]],
                   iota(7,9),
                   [[[9],[10,11]],[[12]]],
                   Vec([13.3,666,105]) );
assertEq( app.data, [1,2,3,4,5,6,7,8,9,10,11,12,13,666,105] );