$ease: (
    in-quad:      cubic-bezier(0.550,  0.085, 0.680, 0.530),
    in-cubic:     cubic-bezier(0.550,  0.055, 0.675, 0.190),
    in-quart:     cubic-bezier(0.895,  0.030, 0.685, 0.220),
    in-quint:     cubic-bezier(0.755,  0.050, 0.855, 0.060),
    in-sine:      cubic-bezier(0.470,  0.000, 0.745, 0.715),
    in-expo:      cubic-bezier(0.950,  0.050, 0.795, 0.035),
    in-circ:      cubic-bezier(0.600,  0.040, 0.980, 0.335),
    in-back:      cubic-bezier(0.600, -0.280, 0.735, 0.045),
    out-quad:     cubic-bezier(0.250,  0.460, 0.450, 0.940),
    out-cubic:    cubic-bezier(0.215,  0.610, 0.355, 1.000),
    out-quart:    cubic-bezier(0.165,  0.840, 0.440, 1.000),
    out-quint:    cubic-bezier(0.230,  1.000, 0.320, 1.000),
    out-sine:     cubic-bezier(0.390,  0.575, 0.565, 1.000),
    out-expo:     cubic-bezier(0.190,  1.000, 0.220, 1.000),
    out-circ:     cubic-bezier(0.075,  0.820, 0.165, 1.000),
    out-back:     cubic-bezier(0.175,  0.885, 0.320, 1.275),
    in-out-quad:  cubic-bezier(0.455,  0.030, 0.515, 0.955),
    in-out-cubic: cubic-bezier(0.645,  0.045, 0.355, 1.000),
    in-out-quart: cubic-bezier(0.770,  0.000, 0.175, 1.000),
    in-out-quint: cubic-bezier(0.860,  0.000, 0.070, 1.000),
    in-out-sine:  cubic-bezier(0.445,  0.050, 0.550, 0.950),
    in-out-expo:  cubic-bezier(1.000,  0.000, 0.000, 1.000),
    in-out-circ:  cubic-bezier(0.785,  0.135, 0.150, 0.860),
    in-out-back:  cubic-bezier(0.680, -0.550, 0.265, 1.550)
);

@function ease($key) {
    @if map-has-key($ease, $key) {
        @return map-get($ease, $key);
    }

    @warn "Unkown '#{$key}' in $ease.";
    @return null;
}

/*---------- USAGE:

.example {
  animation: there-and-back 2.5s ease(in-quad) infinite alternate;
}

*/