(init-mixin params ..body)
  params: an arr
  body: zero or more forms

Defines a mixin initializer method.

This is effectively a wrapper method for init. When a class has the clause (mixin First Second), it has an init method, which is wrapped by Second:init-mixin, which is in turn wrapped by First:init-mixin.

Don't forget to call (@base args) to execute the wrapped initializer!