class relation:
	def __init__(self, *fields):
		self._fields = tuple(fields)
		self._degree = len(self._fields)
	def fields(self):
		return self._fields
	def degree(self):
		return self._degree

class namemap:
	def __init__(self, *pairs):
		self._map = {}
		self._rmap = {}
		for key,val in pairs:
			self._map[key]=val
			self._rmap[val]=key
	def map(self, i):
		return self._map[i]
	def rmap(self, i):
		return self._rmap[i]

nm = namemap(('A','a'), ('B','b'))

print nm._map
print nm._rmap

rel = relation('a', 'b', 'c')
print rel.fields()
print rel.degree()