[mtklein]

what I mean here about NaNs is that from a testing perspective, I want to be able to write a test that expects NaN in the same way that I write other expectations, and you can't do that with ==.

    assert(x == 7);    // fine
    assert(y == NaN);  // never true
    assert(y != y);    // this is what you meant

so this equiv() helper fixes that,

    assert(equiv(x, 7));    // fine
    assert(equiv(y, NaN));  // also fine

now, as far as treating NaNs equivalently, the IEEE 754 float format has a huge number of possible representations of NaN, and if you did something like a bitwise comparison, you might think that 0x7fc00000, 0x7f800001, 0xffc00000, 0x7fc0f00d were all different and not equivalent, but they're all NaNs, and I find that when I'm looking for a NaN, I very rarely care about exactly which one I'm looking at. So checking (x!=x && y!=y) admits any two NaNs as equivalent.