Substitution in C

/* Substitute new for old in tree */
TOKEN subst (TOKEN new, TOKEN old, TOKEN tree)
{  TOKEN tok, last, opnd, ptr;
   if (tree == NULL) return (tree);
   if (tree->tokentype == OPERATOR)
      {  last = NULL;
         ptr = tree->operands;
         tok = copytok(tree);
         while ( ptr != NULL )
            {  opnd = subst (new, old, ptr);
               if (last == NULL)
                       tok->operands = opnd;
                  else last->link = opnd;
               last = opnd;
               ptr = ptr->link;
            }
         return (tok) ;
     	  }
     else if (tree->tokentype == IDENTIFIERTOK
              && strcmp(tree->stringval,
                         old->stringval) == 0)
             return ( copytok(new) );
        else return ( copytok(tree) );
}