SUBROUTINE  VISRB (X,Y,J,M,X1,Y1,N1,X2,Y2,N2,S)

C     [RESTRICTED BOUND]
C     THE BOUND IS RESTRICTED TO THE INTERVAL WHERE THE FIRST
C     FUNCTION IS DEFINED.
C     X(M),Y(M)     ARRAYS FOR THE BOUND OF TWO FUNCTIONS
C     J,M           ACTUAL DIMENSION, MAXIMUM DIMENSION OF X,Y
C     X1(N1),Y1(N1) FIRST  FUNCTION
C     X2(N2),Y2(N2) SECOND FUNCTION
C     S             TYPE OF BOUND (S=1.0,UPPER; S=-1.0,LOWER)
C     [15-MAY-74]

      LOGICAL*1     EQ,VISSL
      VIRTUAL   X(1),Y(1),X1(1),Y1(1),X2(1),Y2(1)

      EQ(X,Y)=ABS(Y-X).LE.1.0E-5

      L=.TRUE.
      J=0
      J1=1
      J2=1
      Z1=X1(J1)
      Z2=X2(J2)
      IF (N1.LE.1)   RETURN
      IF (N2.LE.1)   GO TO 60
      IF (EQ(Z1,Z2)) GO TO 32
      IF (Z1-Z2)     10,32,20

   10 J=MIN0(J+1,M)
      X(J)=Z1
      Y(J)=Y1(J1)
      J1=J1+1
      Z1=X1(J1)
      IF (EQ(Z1,Z2)) GO TO 32
      IF (Z1-Z2)     10,32,32

   20 J2=J2+1
      Z2=X2(J2)
      IF (EQ(Z1,Z2)) GO TO 32
      IF (Z1-Z2)     32,32,20

   30 IF (J1.GT.N1) RETURN
      IF (J2.GT.N2) GO TO 60
      Z1=X1(J1)
      Z2=X2(J2)
   32 Z=AMIN1(Z1,Z2)
      W1=VISLI(Z,X1,Y1,MAX0(J1,2))
      W2=VISLI(Z,X2,Y2,MAX0(J2,2))
      IF (EQ(Z,Z1)) J1=J1+1
      IF (EQ(Z,Z2)) J2=J2+1

      W=S*AMAX1(S*W1,S*W2)
      D=W1-W2
      IF (L.OR.(J.LE.1)) GO TO 42
      IF ((EQ(D,0.0)).OR.(EQ(E,0.0))) GO TO 41
      IF (SIGN(1.0,D).EQ.SIGN(1.0,E)) GO TO 41
      J=MIN0(J+1,M)
      X(J)=U-E*((Z-U)/(D-E))
      Y(J)=WW+(X(J)-U)*((W1-WW)/(Z-U))
   41 IF (VISSL(Z,W,X,Y,J)) GO TO 43
   42 J=MIN0(J+1,M)
   43 X(J)=Z
      Y(J)=W
      L=.FALSE.
      E=D
      U=Z
      WW=W1
      GO TO 30

   60 IF (J1.GT.N1) RETURN
      J=MIN0(J+1,M)
      X(J)=X1(J1)
      Y(J)=Y1(J1)
      J1=J1+1
      GO TO 60

      END