In Ruby, "explicit" visibility modifiers override "implicit" ones. For
example, in the following:
```rb
class C
private
def m1
end
public m2
end
def m3
end
public :m3
end
```
`m1` is private whereas `m2` and `m3` are public.
`Module#private_class_method` takes a symbol representing the name of a
method in the current module scope and makes that module private. This
is similar to `private`, but applies only to class (singleton) methods.
Unlike `private`, it must be called with an argument, and does not
change the ambient visibility for any subsequent method definitions.
class Foo
def public
end
def private1
end
private_class_method :private1
# This alternate form works because method definition
# returns its name as a symbol:
private_class_method def private2
end
end
