isinit - initialize surface descriptor
        iscoeff - get coefficients of surface
         iseval - evaluate the surface at a point
         isfree - free surface descriptor
       islaccum - accumulate surface at a given line number
         islfit - fit the surface at a given line number
       islrefit - refit the surface at a given line number, same columns
       islsolve - solve the surface at a given line number
        islzero - zero the fit for a given line number
        issolve - solve the surface
      isreplace - restore the surface fit
      isresolve - resolve surface, same lines
         issave - save the surface fit
       isvector - fit surface at a set of (x,y) values
         iszero - zero the surface fit