class Tilt::Template

def evaluate(scope, locals, &block)

this method unless they implement the #template_source.
Process the template and return the result. Subclasses should override 
def evaluate(scope, locals, &block)
  source, offset = local_assignment_code(locals)
  source = [source, template_source].join("\n")
  scope.instance_eval source, eval_file, line - offset
end