The readme says:

For example, copy this code snippet to your custom module (change the "foo" in
the function name to be your module's name).

The example code then says 

function foo_corners_init() {}

Following the instructions literally would leave you with:

function mymodule_corners_init(){}

Which won't work. You need: 

function mymodule_init(){}

Need to remove "corners" from example function name.