speedInOut: Reverse easing function: speed in, slow down, speed out

speedInOut.js

// as found on http://stackoverflow.com/questions/7243065/help-with-custom-jquery-easing-function

// supporting math function (not included in the standard javascript Math object)
function sinh(aValue) {
  var myTerm1 = Math.pow(Math.E, aValue);
  var myTerm2 = Math.pow(Math.E, -aValue);
  return (myTerm1-myTerm2)/2;
}

// adds the function to the javascript easing object
$.easing.speedInOut = function(x, t, b, c, d) {
    return (sinh((x - 0.5) * 5) + sinh(-(x - 0.5)) + (sinh(2.5) + Math.sin(-2.5))) /    (sinh(2.5) * 1.82);
};

// works fine like this as an easing function for eg skrollr
function(p) {
    return (sinh((p - 0.5) * 5) + sinh(-(p - 0.5)) + (sinh(2.5) + Math.sin(-2.5))) /    (sinh(2.5) * 1.82);
};