LOGICAL FUNCTION  VISSL (EX,WY,X,Y,I)

C     [STRAIGHT LINE]
C     [15-MAY-74]

      VIRTUAL   X(1),Y(1)

      X1=X(I-1)
      X2=X(I)
      Y1=Y(I-1)
      Y2=Y(I)
      D=(WY-Y1)*(X2-X1)-(EX-X1)*(Y2-Y1)
      VISSL=ABS(D).LT.(1.0E-10)
      RETURN

      END