Nitpick: The if seems a little verbose, it could be reduced to simply one of:
```D
//Result ≡ PI (mod 2PI)
assert(approxEqual(std.math.abs(arg(rec1b)), PI, EPS)));
assert(approxEqual(std.math.cos(arg(rec1b)), -1, EPS)));
assert(approxEqual(std.math.sin(arg(rec1b)), 0, EPS)));
```
Furthermore, if you want to be *very* anal, I *think* the `if` *could* be wrong, in the sense that depending on where the rounding occurs in the call chain, the first result *could* be actually be `-PI`, whereas the second result could be `PI`

Doing this should be the simplest and most correct form, I think?
```D
auto arg1b = arg(rec1b);
assert(approxEqual(arg1b, PI, EPS) || approxEqual(arg1b, -PI, EPS));
```