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+# A class which presents the reverse of a sequence without duplicating it.
+# From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
+
+# It works on mutable or inmutable sequences.
+#
+# >>> for c in Rev( 'Hello World!' ) : sys.stdout.write( c )
+# ... else: sys.stdout.write( '\n' )
+# ... 
+# !dlroW olleH
+#
+# The .forw is so you can use anonymous sequences in init, and still
+# keep a reference the forward sequence. ) 
+# If you give it a non-anonymous mutable sequence, the reverse sequence
+# will track the updated values. ( but not reassignment! - another 
+# good reason to use anonymous values in creating the sequence to avoid
+# confusion. Maybe it should be change to copy input sequence to break
+# the connection completely ? )
+#
+# >>> nnn = range( 0, 3 )
+# >>> rnn = Rev( nnn )
+# >>> for n in rnn: print n
+# ... 
+# 2
+# 1
+# 0
+# >>> for n in range( 4, 6 ): nnn.append( n )	# update nnn
+# ... 
+# >>> for n in rnn: print n	# prints reversed updated values
+# ... 
+# 5
+# 4
+# 2
+# 1
+# 0
+# >>> nnn = nnn[1:-1]
+# >>> nnn
+# [1, 2, 4]
+# >>> for n in rnn: print n	# prints reversed values of old nnn
+# ... 
+# 5
+# 4
+# 2
+# 1
+# 0
+# >>> 
+#
+# WH = Rev( 'Hello World!' )
+# print WH.forw, WH.back
+# nnn = Rev( range( 1, 10 ) )
+# print nnn.forw
+# print nnn
+# 
+# produces output:
+# 
+# Hello World! !dlroW olleH
+# [1, 2, 3, 4, 5, 6, 7, 8, 9]
+# [9, 8, 7, 6, 5, 4, 3, 2, 1]
+# 
+# >>>rrr = Rev( nnn ) 
+# >>>rrr
+# <1, 2, 3, 4, 5, 6, 7, 8, 9>	
+
+from string import joinfields
+class Rev:
+	def __init__( self, seq ):
+		self.forw = seq
+		self.back = self
+	def __len__( self ):
+		return len( self.forw )
+	def __getitem__( self, j ):
+		return self.forw[ -( j + 1 ) ]
+	def __repr__( self ):
+		seq = self.forw
+		if type(seq) == type( [] ) : 
+			wrap = '[]'
+			sep = ', '
+		elif type(seq) == type( () ) : 
+			wrap = '()'
+			sep = ', '
+		elif type(seq) == type( '' ) : 
+			wrap = ''
+			sep = ''
+		else: 
+			wrap = '<>'
+			sep = ', ' 
+		outstrs = []
+		for item in self.back :
+			outstrs.append( str( item ) )
+		return wrap[:1] + joinfields( outstrs, sep ) + wrap[-1:]