double precision function kget_track_err_mass(ktrk)
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*  Returns the mass 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 mass can be written as
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*         M = sqrt(E**2 - Px**2 - Py**2 - Pz**2)
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*   so    dM = (E*dE - Px*dPx - Py*dPy - Pz*dPz) / M
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*   The covariance matrix of the mass is then
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*       sig**2 = A*Vx0*A(t)
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*    where A = (-Px, -Py, -Pz, E) / M
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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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