function once(func) {
	      return before(2, func);
	    }
Creates a function that is restricted to invoking `func` once. Repeat calls
to the function return the value of the first call. The `func` is invoked
with the `this` binding and arguments of the created function.

@static
@memberOf _
@category Function
@param {Function} func The function to restrict.
@returns {Function} Returns the new restricted function.
@example

var initialize = _.once(createApplication);
initialize();
initialize();
// `initialize` invokes `createApplication` once