Expr$ = "1 + 2 * (3 + (4 * 5 + 6 * 7 * 8) - 9) / 10"
      PRINT "Input = " Expr$
      AST$ = FNast(Expr$)
      PRINT "AST =   " AST$
      PRINT "Value = " ;EVAL(AST$)
      END

      DEF FNast(RETURN in$)
      LOCAL ast$, oper$
      REPEAT
        ast$ += FNast1(in$)
        WHILE ASC(in$)=32 in$ = MID$(in$,2) : ENDWHILE
        oper$ = LEFT$(in$,1)
        IF oper$="+" OR oper$="-" THEN
          ast$ += oper$
          in$ = MID$(in$,2)
        ELSE
          EXIT REPEAT
        ENDIF
      UNTIL FALSE
      = "(" + ast$ + ")"

      DEF FNast1(RETURN in$)
      LOCAL ast$, oper$
      REPEAT
        ast$ += FNast2(in$)
        WHILE ASC(in$)=32 in$ = MID$(in$,2) : ENDWHILE
        oper$ = LEFT$(in$,1)
        IF oper$="*" OR oper$="/" THEN
          ast$ += oper$
          in$ = MID$(in$,2)
        ELSE
          EXIT REPEAT
        ENDIF
      UNTIL FALSE
      = "(" + ast$ + ")"

      DEF FNast2(RETURN in$)
      LOCAL ast$
      WHILE ASC(in$)=32 in$ = MID$(in$,2) : ENDWHILE
      IF ASC(in$)<>40 THEN = FNnumber(in$)
      in$ = MID$(in$,2)
      ast$ = FNast(in$)
      in$ = MID$(in$,2)
      = ast$

      DEF FNnumber(RETURN in$)
      LOCAL ch$, num$
      REPEAT
        ch$ = LEFT$(in$,1)
        IF INSTR("0123456789.", ch$) THEN
          num$ += ch$
          in$ = MID$(in$,2)
        ELSE
          EXIT REPEAT
        ENDIF
      UNTIL FALSE
      = num$