Formatter

import std.array : appender;
import std.string : outdent, stripLeft;

auto output = appender!string;
auto writer = writer(formatter(output));

writer.put(["a", "b", "x"]);
writer.put(["a", "b", "y"]);
writer.put(["a", "z"]);
writer.close;

const expected = `
    package a {
        package b as a.b {
            package x as a.b.x {}
            package y as a.b.y {}
        }
        package z as a.z {}
    }
    `;

assert(output.data == outdent(expected).stripLeft);