double precision function kget_track_err_theta(ktrk)
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*  Returns the theta error of a track to the caller.
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*  Input Parameters:
*  ktrk         integer variable
*               Track number
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*  Output Parameters:
*  None
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*  Return value:
*  Error on mass
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*  Other routines:
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*  Notes:
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*   The equation for theta can be written as
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*         cos(theta) = Pz / sqrt(Px**2 + Py**2 + Pz**2)
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*   so    dtheta = (Px*Pz*dPx + Py*Pz*dPy - Pt**2*dPz) / (P**2*Pt)
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*   The covariance matrix of the momentum is then
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*       sig**2 = A*Vx0*A(t)
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*    where A = (Px*Pz, Py*Pz, -Pt**2, 0) / (P**2*Pt)
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*  Author:   Paul Avery      Created:  Tue Aug 26 19:35:08 EDT 1997
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*  Major revisions:
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