c linear interpolation of y(x) in x=x0
      FUNCTION rlin(x0,x,y,n,i0)
      INTEGER n,i0,i,i1
      REAL x(n),y(n),x0
c
      IF (x0.eq.x(1)) THEN
         i0=1
         rlin=y(1)
         GO TO 21
      END IF
      i1=MAX(i0,1)
      DO i=i1,n-1
         IF (x0.eq.x(i+1)) THEN
            i0=i
            rlin=y(i)
            GO TO 21
         END IF
         IF (x(i).le.x0.and.x0.lt.x(i+1)) THEN !increasing
            i0=i
            GO TO 20
         END IF
         IF (x(i+1).le.x0.and.x0.lt.x(i)) THEN !decreasing
            i0=i
            GO TO 20
         END IF
      END DO
 20   IF (i0.eq.0) THEN
         rlin=0.
      ELSE
         rlin=y(i0)+(y(i0+1)-y(i0))*(x0-x(i0))/(x(i0+1)-x(i0))
      END IF
 21   RETURN
      END