SUBROUTINE  VISHH (X0,T0,B0,N0,X,Y,N,I,PL)

C     [HALF HORIZON]
C     SOME OF THE HIDDEN LINE SUBROUTINES EMPLOY MORE THAN ONE HORIZON,
C     WHICH MEANS THAT THE ARRAYS CONTAINING THESE HORIZONS MUST APPEAR
C     AS EXPLICIT ARGUMENTS IN THE UPDATING SUBROUTINES. NEVERTHELESS,
C     THREE OF THE ARGUMENTS OF VISBO ARE NOTHING BUT WORKING ARRAYS
C     WHICH CAN STILL BE REMOVED FROM THE CALLING PROGRAMS IF THEY ARE
C     PLACED IN AN INTERMEDIATE SUBROUTINE SUCH AS THIS ONE.
C     X0(N0)  ARRAY OF ARGUMENTS FOR THE HORIZON
C     T0(N0)  ARRAY OF VALUES OF THE UPPER HORIZON
C     B0(N0)  ARRAY OF VALUES OF THE LOWER HORIZON
C     X(N)    ARRAY OF ARGUMENTS
C     Y(N)    ARRAY OF FUNCTION VALUES
C     I       PEN DIRECTION (1=FORWARD, -1=BACKWARD)
C     PL      PEN MOVEMENT SUBROUTINE
C     [29-MAY-74]

      EXTERNAL    PL
      VIRTUAL   X(1),Y(1)
      VIRTUAL   X0(1),T0(1),B0(1)
      VIRTUAL   X1(701),T1(701),B1(701)
      DATA        M/701/

      CALL VISBO (X1,T1,B1,M,X0,T0,B0,N0,X,Y,N,I,PL)
      RETURN

      END