If the ev module is enabled, and this fiber is used as an argument to `ev/go`, these "blocked" signals will result in messages being sent to the supervisor channel. The default sigmask is :y. For example,

 (fiber/new myfun :e123)

blocks error signals and user signals 1, 2 and 3. The signals are as follows:

* :a - block all signals
* :d - block debug signals
* :e - block error signals
* :t - block termination signals: error + user[0-4]
* :u - block user signals
* :y - block yield signals
* :w - block awai