SUBROUTINE cmc_POSMP(H,A,V,O,PN,E1,E2)
C*************************************************************
C--> THIS SUBROUTINE FINDS THE POINT AT WHICH THE LINE DEFINED
C    BY A AND V (A=POINT; V=DIRECTION) INTERSECTS THE PLANE
C    DEFINED BY O AND PN (  (Q-O) DOT PN = 0 FOR ALL Q IN THE PLANE)
C    IT THEN PROJECTS ONTO THE UNIT VECTORS E1 AND E2.
C
C--> IN OTHER WORDS, GIVEN A POSITION AND DIRECTION AT A KNOWN Z,
C    THIS SUBROUTINE CALCULATES XM,YM - X,Y IN THE MIRROR PLANE
C*************************************************************
      SAVE
      DIMENSION H(3),A(3),V(3),O(3),E1(3),E2(3),PN(3)
      OA1=O(1)-A(1)
      OA2=O(2)-A(2)
      OA3=O(3)-A(3)
      AL=(OA1*PN(1)+OA2*PN(2)+OA3*PN(3))
      AL=AL/(V(1)*PN(1)+V(2)*PN(2)+V(3)*PN(3))
      H(1)=-OA1+AL*V(1)
      H(2)=-OA2+AL*V(2)
      H(3)=-OA3+AL*V(3)
      H(1)=H(1)*E1(1)+H(2)*E1(2)+H(3)*E1(3)
      H(2)=H(1)*E2(1)+H(2)*E2(2)+H(3)*E2(3)
      H(3)=0.
      RETURN
      END