DOUBLE PRECISION FUNCTION KSGPHI(PW, VP, MVP)
#endif
#if !defined(CLEO_DOUBLE)
      REAL FUNCTION KSGPHI(PW, VP, MVP)
#endif
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Returns the error in phi for a track in the W parameterization.
C
C  PW        real array (read)
C            Track parameters in W form (at least 3-vector part)
C
C  VP        real array (read)
C            Covariance matrix
C
C  MVP       integer variable (read)
C            First dimension of VP (must be at least 3)
C
C
C   The equation for phi can be written as
C
C         cot(phi) = Px / Py
C
C   so    dphi = (-Py*dPx + Px*dPy) / (Px**2 + Py**2)
C
C   The covariance matrix of the momentum is then
C
C       sig**2 = A*Vx0*A(t)
C
C    where A = (-Py, Px, 0, 0) / (Px**2 + Py**2)
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>