## AIRES library reference: xvfromxs.

```c
c     Routine directly callable from C/C++ (it could be necessary to
c     append an underscore, "_", to its name depending on the C/C++
c     compiler used).
c
c
function xvfromxs(xslant0, xs0, cozenith, zcground)
c
c     Evaluating the vertical depth corresponding to the end point of a
c     slanted path (in g/cm2) along an axis starting at altitude
c     zcground, and inclination given by cozenith (cosine of the zenith
c     angle). The path corresponds to the "segment" whose
c     ends possess slant depths xslant and xs0, respectively. If
c     xs0 = 0 then the path up to the top of the atmosphere is
c     evaluated. The earth's curvature is taken into account to perform
c     the calculations.
c
c     The slanted path is defined via
c
c                    point2
c          Xslant = INTEGRAL'  rho(zv) dl
c                    point1
c
c     where rho(zv) is the density of the medium, which is supposed to
c     depend only on zv, the vertical altitude. The prime in the
c     integral indicates that it is done along the specified axis.
c
c     The atmospheric model must be properly initialized before
c     invoking this routine.
c
c     Written by: S. J. Sciutto, La Plata 2003, 2019.
c
c
c     Parameters:
c     ==========
c
c     xslant0......... (input, double precision) Slant depth of
c                      the path's end point 1 (g/cm2).
c     xs0............. (input, double precision) Slant depth of
c                      the path's end point 2 (g/cm2). If xs0 is zero
c                      then the path is evaluated up to the top of the
c                      atmosphere.
c     cozenith........ (input, double precision) Cosine of the axis'
c                      zenith angle. Must be in the range (0, 1].
c     zcground........ (input, double precision) Central altitude of
c                      the ground level, which defines the intersection
c                      between the slanted axis and the z-axis.
c
c     Return value: (double precision) The vertical depth of point 1,
c     ============  in g/cm2. The returned value is zero on error.
c
c
c<--->
```

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