img-retina($file-1x, $file-2x, $width-1x, $height-1x) {\n  background-image: url($file-1x);\n\n  // Autoprefixer takes care of adding -webkit-min-device-pixel-ratio and -o-min-device-pixel-ratio,\n  // but doesn't convert dppx=>dpi.\n  // There's no such thing as unprefixed min-device-pixel-ratio since it's nonstandard.\n  // Compatibility info: https://caniuse.com/css-media-resolution\n  @media only screen and (min-resolution: 192dpi), // IE9-11 don't support dppx\n    only screen and (min-resolution: 2dppx)...

R to 2^(_W * n) R mod m (aka from one to R in // the Montgomery domain, meaning we can use Montgomery multiplication now). // We could do that by doubling _W * n times, or with a square-and-double // chain log2(_W * n) long. Turns out the fastest thing is to start out with // doublings, and switch to square-and-double once the exponent is large // enough to justify the cost of the multiplications. // The threshold is selected experimentally as a linear function of n. threshold := n / 4 // We calculate...