code: function once(callback) {
    var called = false;
    var cache;
    return function () {
      if (!called) {
        cache = callback.apply(undefined, arguments);
        called = true;
      }
      return cache;
    };
  }
docs: @function
  @name once
  @memberof VisSense.Utils
  
  @param {function} callback The function to proxy.
  
  @returns {function} A proxy function that will only be invoked once.
  
  @description Returns a function that is restricted to invoking `callback`
  once. Repeat calls to the function return the value of the first call.
  
  @example
  
  var calculateExpensiveNumber = function() { ... };
  var expensiveNumber = once(calculateExpensiveNumber);
  
  var a = expensiveNumber() * 3.1415 + expensiveNumber();
  // => exensiveNumber is actually invocing `calculateExpensiveNumber` only once