Error using `equal` with various string types

[Andrej Mitrovic]

It seems this doesn't work:

import std.algorithm;
auto b1 = qual(["foo"d], ["foo"]);
auto b2 = equal(["foo"d.dup], ["foo"]);

It's due to a constraint failure:
is(typeof(r1.front == r2.front))

The first call is:
immutable(dchar)[]
string

The second:
dchar[]
string

Anyway can we make `equal` work with these? It would be really useful
if it worked.