Property

Property p;
assert(!p.isCallableValue);
p = 42;
assert(!p.isCallableValue);
assert(p == 42);
assert(p.get!int == 42);

p = () @system => Variant(1337);
assert(p.isCallableValue);
assert(p.get!int() == 1337);

auto prop1 = Properties("first");
prop1["foo"] = "hello";

auto prop2 = Properties("second");
prop2["foo"] = () @system => Variant(prop1["foo"].get!string ~ " world!");

assert(prop2["foo"].isCallableValue);
assert(prop2["foo"].get!string() == "hello world!");
prop1["foo"] = "goodbye cruel";
assert(prop2["foo"].get!string() == "goodbye cruel world!");