def instance(rolecls, subj):
    """
    Apply the role class to the subject. This is the default role assignment
    method.

    >>> class Person(object):
    ...     def __init__(self, name): self.name = name
    ...     def am(self): print self.name, 'is'
    >>> class Biker(object):
    ...     __metaclass__ = RoleType
    ...     def bike(self): print self.name, 'bikes'
    ...
    >>> person = Person('Joe')
    >>> biker = Biker(person, method=instance)
    >>> biker # doctest: +ELLIPSIS
    <roles.role.Person+Biker object at 0x...>
    >>> person # doctest: +ELLIPSIS
    <roles.role.Person+Biker object at 0x...>
    >>> person is biker
    True
    >>> biker.bike()
    Joe bikes
    """
    subj.__class__ = rolecls
    return subj
