procedure S;

  begin
    if nexttoken = "if"
      then begin
        scan;
        E;
        if nexttoken <> "then" 
          then error
          else begin
            scan;
            S;
            if nexttoken = "else"
               then begin
                 scan;
                 S;
               end;
          end;
       end
       else begin
         V;
         if nexttoken <> ":="
           then error
           else begin
             scan;
             E;
           end;
       end;

end S;



procedure V;

  begin
    if nexttoken <> "id"
       then error
       else begin
         scan;
         if nexttoken = "("
           then begin
             scan;
             E;
             if nexttoken <> ")"
               then error
               else scan;
           end;
       end;

end V;



procedure E;
  
  begin
    T;
    if nexttoken = "+"
      then begin
        scan;
        E;
      end
      else scan;

end E;



procedure T;

  begin
    F;
    if nexttoken = "*"
      then begin
        scan;
        T;
      end
      else scan;

end T;



procedure F;

  begin
    if nexttoken = "("
      then begin
        scan;
        E;
        if nexttoken <> ")"
          then error
          else scan;
      end
      else V;

end F;