functions in drat.i - u

             update_mesh, mesh, rt, zt  
          or update_mesh, mesh, rt, zt, ireg  
 
     updates the opaque MESH object to reflect a new RT, ZT, and  
     (optionally) IREG.  The boundary edges are recomputed and stored  
     in MESH, as well.  

builtin function, documented at i0/drat.i   line 1086