The suffix "Task" in class names like "RDocTask" etc. is confusing
because an RDocTask isn't actually a task. It is a class whose objects
create several tasks inside the Rakefile they're created in. For
example, "RDocTask" defines a task :rdoc for generating the
documentation, a task :clobber_rdoc to delete the generated
documentation, and a task :clobber to first delete, then re-generate
the documentation.

Tasks are the things that can be directly created using "task".

Multiple invocations of "task" with the same task name modify the same
task instead of creating a new one each time.


--internal--

rake.rb:

Tasks are instances of Task (or one of its subclasses).

A task's prerequisites are in @prerequisites (list of strings, not
interpreted immediately).

A task's actions (the actions given as blocks to "task") are in
@actions.

Task.needed? tells whether or not the task must be run.

A task is run using Task.invoke. Task.invoke first runs all
prerequisites via "@prerequisites.each { |n| Task[n].invoke }" (see
below for Task.[]), then calls self.execute if self.needed?.

The needed? implementation in Task returns true.

The execute implementation in Task runs all @actions (in order of
definition). If no actions have been defined, [TODO: rules].

Task.[](task_name): If a task task_name was defined explicitly, return
it. Else, if a rule's pattern (the thing before the "=>" in the
definition) matches task_name, and an accordingly named source file
exists, on-the-fly create and return the file task: file task_name =>
accordingly_named_source_file {<the rule's block>} (the "source"
attribute of that task is set to the
accordingly_named_source_file). Else, if a file named task_name
exists, return a file(task_name) task. Else, fail.

Rules can only have a single prerequisite :(

e.g.
rule '.foo' => ['.bar','baz']
=> RuntimeError: Too many dependents specified in rule .foo: [".bar", "baz"]


file(name, &block) calls FileTask.define_task(name,&block).

FileTask (< Task) only overrides needed? (and timestamp): A FileTask
is needed if its file doesn't exist, or is not younger than all its
prerequisites (if there are no prerequisites and the file exists, the
FileTask is not needed).

Task.timestamp returns the maximum of the task's prerequisites's
timestamps or, if that isn't defined, the current time.