import std.stdio, std.algorithm, std.array;

auto mode(T)(T[] items) pure /*nothrow @safe*/ {
    int[T] aa;
    foreach (item; items)
        aa[item]++;
    immutable m = aa.byValue.reduce!max;
    return aa.byKey.filter!(k => aa[k] == m);
}

void main() /*@safe*/ {
    auto data = [1, 2, 3, 1, 2, 4, 2, 5, 3, 3, 1, 3, 6];
    writeln("Mode: ", data.mode);

    data ~= 2;
    writeln("Mode: ", data.mode);
}