Compute approximate effect of solar phase angle on apparent position
of occulted or eclipsed body in mutual event.  Correction is applied
to that body's position.  The correction is fr*rhb*(1-cos(alpha)),
where fr is about .5 (fr depends just on the projected minimum
separation and the radius of the other body).

ibsi=    index of obscured body coordinates in xscs (saved as ibs)
xni=     vector from earth observer to that body (saved as xn)
and xn(4),xn(5) are the distance squared and distance
xa,xb=   vectors from proper center to obscurer, obscured
rhai,rhbi=respective radii (saved as rha,rhb)

make corrections to partials of obscured body
called from cpartc
derpl and dersc contain partials of observed and other body,
respectively (or else garbage to be ignored).