/* 1 2 RCcell(r,c) == value of cell from row r column c
*/
double nf_RCcell(const Node *n, const Cell *c)
{
  Cell x;  Node *r = Right(n);

  x.row_fixed = x.col_fixed = 0;

  x.row = eval_tree( r, c);
  x.col = eval_tree( r->next, c);

  if( check_cell( &x)) return 0;

  SS *cp = &ss[x.row][x.col];

  return eval_cell( cp, &x);
}