PROGRAM ch2407a
IMPLICIT NONE
include 'block1'
     PRINT *,'calling subroutine sub1'
!    sub1 sets up values for A,B,C,root,I,J,K,L
	 CALL sub1
	 PRINT *,'on return from sub 1'
	 PRINT *,'A,B,C,root = ',A,B,C,root
	 PRINT *,'I,J,K,L = ',I,J,K,L
!
!    sub2 is only supposed to use the values A,B,C to calculate root
!    but by mistake it changes L 
!
     CALL sub2
	 PRINT *,'on return from sub 2'
	 PRINT *,'A,B,C,root = ',A,B,C,root
	 PRINT *,'I,J,K,L = ',I,J,K,L

END PROGRAM ch2407a


SUBROUTINE sub1
IMPLICIT NONE
include 'block1'

   PRINT *,'input values for A,B,C'
   READ *,A,B,C
   root=-999.0
   PRINT *,'input values for I,J,K,L'

   READ *,I,J,K,L
END SUBROUTINE sub1

SUBROUTINE sub2
IMPLICIT NONE
REAL:: temp
include 'block1'
   temp = B*B-4.0*A*C
   IF(temp >= 0.0) THEN
      root=(-B+SQRT(temp))/(2.0*A)
   ELSE
      root =0.0
   ENDIF
!
! this is a mistake but not picked up when using a common block
!
   L=I+J+K
END SUBROUTINE sub2