`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
