PROGRAM chain

c	Adds chain identifier to residue number

	CHARACTER*132 	line

	WRITE (6, 1000) 'Enter filename for input file:'
1000	FORMAT (a)
	READ (5, 1000) line
	OPEN (unit=1, status='OLD', file=line, readonly, form='FORMATTED')
	WRITE (6, 1000) 'Enter filename for output file:'
	READ (5, 1000) line
	OPEN (unit=2, status='UNKNOWN', form='FORMATTED', file=line,
	1	carriagecontrol='LIST')

c	Loop

	DO WHILE (.TRUE.)
	  READ (1, 1000, end=99) line
	  IF (line(1:4) .EQ. 'ATOM' .OR. line(1:6) .EQ. 'HETATM') 
	1	line(73:73) = line(22:22)
	  line(22:22) = ' '
	  WRITE (2, 1000) line(1:76)
	endDO

99	CLOSE (unit=1)
	CLOSE (unit=2)

	end